REF: remove libreduction.SeriesGrouper
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

cc @bashtage per comment https://github.com/pandas-dev/pandas/pull/37468#discussion_r706100798 I'm taking your word for it that the change this makes in the crosstab test is correct.  Does this represent a bugfix?