CLN: Remove `counts` Argument from Cython GroupBy Aggregations
All of the Cython aggregation functions accept `counts` as their second argument. In theory, it looks as if this parameter could assist in the handling of `NaN` values within a group, as dictated by the below lines of code:

https://github.com/pandas-dev/pandas/blob/e022d9a0886b245afe4af4cc43904a112cd00882/pandas/core/groupby.py#L2398

~~However, `self._filter_empty_groups` from what I can tell is always `False`. The only place this really gets exposed is in the `BinGrouper` constructor, but that is an internal class and I don't see any invocations of it that override the default value, essentially making the code linked above a `noop`.~~

I'm assuming that is part of an abandoned implementation, and if that's the case there's a decent amount of code cleanup / simplification that can be had between `groupby.py` and `groupby_helpers.pxi.in` if we remove `counts`