Remove offset/DTI caching (disabled since 0.14
- [x] closes #23080
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/26e36018a35dbf9d1f6f1d34cf974cbbf03617d6/pandas/core/arrays/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/26e36018a35dbf9d1f6f1d34cf974cbbf03617d6/pandas/core/indexes/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/tests/tseries/offsets/test_offsets.py`](https://github.com/jbrockmendel/pandas/blob/26e36018a35dbf9d1f6f1d34cf974cbbf03617d6/pandas/tests/tseries/offsets/test_offsets.py) !

 - There are no PEP8 issues in the file [`pandas/tseries/offsets.py`](https://github.com/jbrockmendel/pandas/blob/26e36018a35dbf9d1f6f1d34cf974cbbf03617d6/pandas/tseries/offsets.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23118?src=pr&el=h1) Report
> Merging [#23118](https://codecov.io/gh/pandas-dev/pandas/pull/23118?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2f05cc8580ef1be09c88d32f5924b7762b619729?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23118/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23118?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23118      +/-   ##
==========================================
+ Coverage   92.13%   92.14%   +0.01%     
==========================================
  Files         170      170              
  Lines       51078    51016      -62     
==========================================
- Hits        47061    47009      -52     
+ Misses       4017     4007      -10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.56% <100%> (ø)` | :arrow_up: |
| #single | `42.3% <37.5%> (+0.02%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23118?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/23118/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.27% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23118/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.49% <100%> (+0.85%)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23118/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `97.38% <100%> (+0.43%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23118?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/23118?src=pr&el=footer). Last update [2f05cc8...6f5648f](https://codecov.io/gh/pandas-dev/pandas/pull/23118?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I agree this is nice to remove. but just to be sure, can you run an asv on date/time types and check that it is really not used anywhere. also add a whatsnew note (put in api breaking changes, even though it has long been disabled) > also add a whatsnew note (put in api breaking changes, even though it has long been disabled)

OK.  What do I say has changed? just say that the attribute has been removed; it’s just a notice that a change was made (though is not actually public folks sometimes rely on this) > just say that the attribute has been removed

Done.  asvs showed no change. lgtm merge on green