Removing an un-needed conditional in np_datetime_strings.c

- [x] closes #21422
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

PS: I was not able to build this properly on my setup, so would wait for the test builds to run.
Also, in which version should I put a whatsnew entry in?