CLN: remove deprecated infer_dst keyword
This has come up a few times during the docstring improvements: we still had the `infer_dst` deprecations left over (deprecated in 0.15)
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20490?src=pr&el=h1) Report
> Merging [#20490](https://codecov.io/gh/pandas-dev/pandas/pull/20490?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/402ad45da3f1c44da6594a773e5ba567ee1966a5?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20490/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20490?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20490      +/-   ##
==========================================
- Coverage   91.85%   91.85%   -0.01%     
==========================================
  Files         152      152              
  Lines       49235    49232       -3     
==========================================
- Hits        45224    45221       -3     
  Misses       4011     4011
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.24% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.88% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20490?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/20490/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.73% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20490/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/20490/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.61% <100%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20490?src=pr&el=footer). Last update [402ad45...bedfb2f](https://codecov.io/gh/pandas-dev/pandas/pull/20490?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jorisvandenbossche 