CLN: Remove u and u_safe from pandas.compat
- [X] xref #25725
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Another incremental step in removing Python 2.  Only non-standard changes were to `pandas/tests/test_strings.py` where I deleted a bunch of code, as removing `u` resulted in duplicate test scenarios.