DEPR/CLN: Remove how keyword from df.rolling() etc.
- [x] xref #18601 and #11603
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The ``how`` parameter of df.rolling/expanding/ewm related methods was deprecated in 0.18 (#11603). This PR removes the parameter from the code base.

This PR is a continuation of #18601 (removed the ``freq`` parameter). Next up will be to remove ``pd.stats.*``).
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18668?src=pr&el=h1) Report
> Merging [#18668](https://codecov.io/gh/pandas-dev/pandas/pull/18668?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/695e89345a46c0cab06c479c83007f5adbb70ad5?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18668/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18668?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18668      +/-   ##
==========================================
- Coverage   91.59%   91.55%   -0.05%     
==========================================
  Files         153      153              
  Lines       51212    51212              
==========================================
- Hits        46908    46887      -21     
- Misses       4304     4325      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.41% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.67% <50%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18668?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/18668/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.31% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18668/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/18668/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/18668/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/18668?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/18668?src=pr&el=footer). Last update [695e893...2f4ba4e](https://codecov.io/gh/pandas-dev/pandas/pull/18668?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 All green. thanks @topper-123 