remove unused time conversion funcs
Takes the place of #17708, removing funcs instead of moving them.

Had to cpdef `pydt_to_i8` because the func it is replacing in `_libs.index` was cdef'd.

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