CI: delete unused import in groupby/groupby.py
Fixes a `flake8` bug on current master:
```
(pandas-dev) andrewwieteska@Andrews-MacBook-Pro pandas % flake8 pandas/core/groupby/groupby.py
pandas/core/groupby/groupby.py:53:1: F401 'pandas.core.dtypes.cast.maybe_cast_result' imported but unused
```