REF: simplify ohlc
Or more accurately, simplify BaseGroupBy._cython_agg_general by not having ohlc go through it.  This will in turn allow us to simplify SeriesGroupBy._wrap_aggregated_output.

This will have a merge conflict with #41066.  Doesn't matter which order they are merged in.
merged the other one, pls rebase rebased + greenish (unrelated pyarrow failure thats failing on everything)