DEPR: Deprecate as_index in groupby
Should we deprecate using `as_index` in groupby? As the docs say, using `as_index=False` should only impact reductions. Here, my expectation is that a line such as

    df.groupby(keys, as_index=False).op(...)

should always be equivalent to

    df.groupby(keys, as_index=True).op(...).reset_index()

It seems quite easy for the user to do so if they desire, and this would simplify the groupby internals. 