DEPR/CLN: Remove freq parameters from df.rolling/expanding/ewm
-- [x ] tests added / passed
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x ] whatsnew entry

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

After this PR, I will remove the ``how`` parameter and lastly the ``pd.rolling_*``, ``pd.expanding_*`` and ``pd.ewm_*`` will be removed (AKA ``pd.stats.*``). By removing ``freq`` and ``how``before ``pd.stats`` I think it will be easier to clean up `` pandas/tests/test_window.py``, as ATM these three issues are not very cleanly separated in that test module.

In some test in ``test_window.py::TestMoments`` there is a bit of resampling going on, as I've moved ``freq`` stuff from ``rolling`` into a prior ``df.resample`` step. These are tests for ``how`` and will be removed once ``how`` is removed (unless the tests good for testing the windows functions, I'm not completely sure ATM, but will look into it when I reach that point).

Additionally (and unrelated), in `` pandas/tests/test_window.py`` there are checks for numpy>=1.8 and >=1.9. These checks are no longer necessary, as numpy 1.9 is the current minium version, so they're removed,.