REF: date arg not reachable in DTI._maybe_cast_slice_bound
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Tracked all the places where _maybe_cast_slice_bound is called and concluded that stdlib date objects are [update: almost] always handled by caller, so don't need to be handled in _maybe_cast_slice bound.  Removing that special-casing (this PR) will allow us to share the method between DTI/TDI/PI (next PR)