CLN: Remove unused code in groupby.py
```bash
williams-mbp:pandas williamayd$ vulture pandas | grep "pandas/core/groupby.py"
pandas/core/groupby.py:947: unused function '_index_with_as_index' (60% confidence)
pandas/core/groupby.py:2280: unused function '_max_groupsize' (60% confidence)
pandas/core/groupby.py:2927: unused attribute '_should_compress' (60% confidence)
pandas/core/groupby.py:4940: unused class 'NDArrayGroupBy' (60% confidence)
pandas/core/groupby.py:4996: unused class 'ArraySplitter' (60% confidence)
```

Grepping the pandas directory for these items individual yields no uses outside of their initial declaration. Should be safe to remove and start cleaning up the module