CLN/TST: Remove agg test from groupby
- [x] 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

Removed test is duplicated of

https://github.com/pandas-dev/pandas/blob/e9d0a58c4152a16153dfb330164991d37fdd02dc/pandas/tests/apply/test_frame_apply.py#L1398-L1406

and

https://github.com/pandas-dev/pandas/blob/e9d0a58c4152a16153dfb330164991d37fdd02dc/pandas/tests/apply/test_series_apply.py#L410-L416