CLN: Resample time bin cruft
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Passes locally. Removes some old code for determining the time bins for resample.
Hello @mroeschke! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/resample.py`](https://github.com/mroeschke/pandas/blob/3b217cf6a14d87985aa8e1d12dddc389e04fe3c1/pandas/core/resample.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24347?src=pr&el=h1) Report
> Merging [#24347](https://codecov.io/gh/pandas-dev/pandas/pull/24347?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6111f645c5adc5bdcd3810b4112392bda3583d59?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/24347/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24347?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24347      +/-   ##
==========================================
+ Coverage   92.29%   92.29%   +<.01%     
==========================================
  Files         162      162              
  Lines       51808    51796      -12     
==========================================
- Hits        47814    47805       -9     
+ Misses       3994     3991       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <100%> (ø)` | :arrow_up: |
| #single | `42.99% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24347?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/24347/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `97.21% <100%> (+0.45%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24347?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/24347?src=pr&el=footer). Last update [6111f64...3b217cf](https://codecov.io/gh/pandas-dev/pandas/pull/24347?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/24347?src=pr&el=h1) Report
> Merging [#24347](https://codecov.io/gh/pandas-dev/pandas/pull/24347?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6111f645c5adc5bdcd3810b4112392bda3583d59?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/24347/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24347?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24347      +/-   ##
==========================================
+ Coverage   92.29%   92.29%   +<.01%     
==========================================
  Files         162      162              
  Lines       51808    51796      -12     
==========================================
- Hits        47814    47805       -9     
+ Misses       3994     3991       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <100%> (ø)` | :arrow_up: |
| #single | `42.99% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24347?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/24347/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `97.21% <100%> (+0.45%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24347?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/24347?src=pr&el=footer). Last update [6111f64...3b217cf](https://codecov.io/gh/pandas-dev/pandas/pull/24347?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke love removing code!