CLN: remove legacy datetime support in io.pytables
also removed the `inferred_type == "unicode"` branch, as `lib.infer_dtype` never returns unicode anymore

- [x] closes #19475
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

happy to get rid of this!