DEPR/CLN: Remove pd.rolling_*, pd.expanding* and pd.ewm*
- [x] xref #18601, #18668 and #11603
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This PR removes ``pd.rolling_*``,`` pd.expanding_*`` ``pd.ewm*`` and the related ``pd.stats`` subpackage from the code base.

I have some doubts about some stuff:
* there are modules ``doc.plots.stats.*`` that I'm not sure of if they should be changed or removed. Currently they're not touched.
* In ``asv_bench.benchmarks.gil.py`` there are calls to ``pd.rolling_*`` etc. I've changed this to be method-based.
* ``pandas.tests.test_windows.py`` I felt was a bit complex to clean up.

I would appreciate inputs.