API: Remove CalendarDay
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Given that we want to ship 0.24.0 soon and that converting `'D'` and `Day` to always act as calendar day warrants more game-planning, this PR just simply removes `CalendarDay` and reverts `'D'` and `Day` to their prior behavior. 
Hello @mroeschke! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/core/arrays/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/resample.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/core/resample.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/datetimes/test_date_range.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tests/indexes/datetimes/test_date_range.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/datetimes/test_timezones.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tests/indexes/datetimes/test_timezones.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/timedeltas/test_timedelta_range.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tests/indexes/timedeltas/test_timedelta_range.py) !

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_datetime_index.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tests/resample/test_datetime_index.py) !

 - There are no PEP8 issues in the file [`pandas/tests/resample/test_period_index.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tests/resample/test_period_index.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_timezones.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tests/series/test_timezones.py) !

 - There are no PEP8 issues in the file [`pandas/tests/tseries/offsets/test_offsets.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tests/tseries/offsets/test_offsets.py) !

 - There are no PEP8 issues in the file [`pandas/tests/tseries/offsets/test_ticks.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tests/tseries/offsets/test_ticks.py) !

 - There are no PEP8 issues in the file [`pandas/tseries/offsets.py`](https://github.com/mroeschke/pandas/blob/18dabb836189e3359efc00ed7786b6cb2846dbcf/pandas/tseries/offsets.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=h1) Report
> Merging [#24330](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/216986d4691297d5cfec33b5c62be7890b9a54d7?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24330/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24330      +/-   ##
==========================================
+ Coverage   92.28%   92.28%   +<.01%     
==========================================
  Files         162      162              
  Lines       51831    51813      -18     
==========================================
- Hits        47830    47816      -14     
+ Misses       4001     3997       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.68% <100%> (ø)` | :arrow_up: |
| #single | `42.98% <61.53%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.95% <ø> (+0.11%)` | :arrow_up: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.76% <100%> (+0.18%)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.24% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.58% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=footer). Last update [216986d...18dabb8](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=h1) Report
> Merging [#24330](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/216986d4691297d5cfec33b5c62be7890b9a54d7?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24330/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24330      +/-   ##
==========================================
+ Coverage   92.28%   92.28%   +<.01%     
==========================================
  Files         162      162              
  Lines       51831    51817      -14     
==========================================
- Hits        47830    47819      -11     
+ Misses       4001     3998       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <100%> (ø)` | :arrow_up: |
| #single | `42.99% <61.53%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.95% <ø> (+0.11%)` | :arrow_up: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.76% <100%> (+0.18%)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.24% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `93.09% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/24330/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `94.82% <0%> (+0.06%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=footer). Last update [216986d...e0d5bfe](https://codecov.io/gh/pandas-dev/pandas/pull/24330?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke 