REF/TYP: use OpsMixin for StringArray
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

its already mixed in, this just removes the leftover _create_arithmetic_method usage.

This is the last one outside of tests, for which we still have DecimalArray to do.