CLN: remove unnecessary _validate_foo methods
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

since DatetimeLike and Categorical no longer special-case these, we can inline them.
i guess the windows failures are spurious