REF: unnecessary method sharing with PeriodIndex
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] 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
- [ ] whatsnew entry

this is just moving things around (in the same file) + typing? yes, and remove a handful of is_period_dtype checks