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.*``).