CLN: Remove .to_datetime methods
Removes the following .to_datetime methods

* `Index.to_datetime`
* `Timestamp.to_datetime`
* `PeriodIndex.to_datetime`
* `DatetimeIndex.to_datetime`

All were deprecated in 0.19.0

xref #8254, #14096, #14113
Oops, I forgot some `flake8` stuff before pushing.  Will fix those later today. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18648?src=pr&el=h1) Report
> Merging [#18648](https://codecov.io/gh/pandas-dev/pandas/pull/18648?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/13f6267207dd1f140b12d7718277508eff5c1efb?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18648/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18648?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18648      +/-   ##
==========================================
- Coverage   91.59%   91.58%   -0.02%     
==========================================
  Files         153      153              
  Lines       51221    51236      +15     
==========================================
+ Hits        46917    46924       +7     
- Misses       4304     4312       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.44% <ø> (ø)` | :arrow_up: |
| #single | `40.68% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18648?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.91% <ø> (-0.04%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.68% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.43% <ø> (-0.02%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.9% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.37% <0%> (+0.06%)` | :arrow_up: |
| [pandas/stats/moments.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL3N0YXRzL21vbWVudHMucHk=) | `71.19% <0%> (+0.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18648?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/18648?src=pr&el=footer). Last update [13f6267...c4977d5](https://codecov.io/gh/pandas-dev/pandas/pull/18648?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/18648?src=pr&el=h1) Report
> Merging [#18648](https://codecov.io/gh/pandas-dev/pandas/pull/18648?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/13f6267207dd1f140b12d7718277508eff5c1efb?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18648/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18648?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18648      +/-   ##
==========================================
- Coverage   91.59%   91.55%   -0.05%     
==========================================
  Files         153      153              
  Lines       51221    51205      -16     
==========================================
- Hits        46917    46880      -37     
- Misses       4304     4325      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.41% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.67% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18648?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.9% <ø> (-0.05%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.68% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.43% <ø> (-0.02%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.44% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18648/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18648?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/18648?src=pr&el=footer). Last update [13f6267...c4977d5](https://codecov.io/gh/pandas-dev/pandas/pull/18648?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 All is green, so merging.