CLN: Remove StringMixin from pandas.core.computation
- [x] xref #25725
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

StringMixin is not really useful anymore and only functions to reverse the calling order of ``__str__`` and ``__repr__``, which is now not-standard in Python3.

This removes the usage of StringMixin in pandas.core.computation. Followups will remove the class from other modules.
