REF: Remove Apply.get_result
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

Slightly simpler and allows typing of the result that we get out of Apply. Having both apply and agg go through get_result would mean typing the result as `Union[FrameOrSeriesUnion, Tuple[bool, Optional[FrameOrSeriesUnion]]]`