CLN: Remove _axis from apply
- [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

The regex `[\W]_axis` is showing this removes the only occurances of the variable `_axis` throughout all of `pandas.core`. It is only set in groupby in a branch where is must be 0, and is only used to check that it is 0.