CI: Deduplicate linting checks

[dsaxton]

#36471 added some linting checks (e.g., black, flake8) to a pre-commit workflow which are duplicated in https://github.com/pandas-dev/pandas/blob/master/ci/code_checks.sh. Once we feel good that the pre-commit checks are right we should go ahead and delete them from code_checks.sh.

cc @MarcoGorelli

[rhshadrach]

I often like to run some of these checks via code_checks.sh even before committing, typically black, isort, and flake8. The first two aren't an issue for me since they don't require command line options. However flake8 requires a lot, is there an easy way to manually run flake8 without doing it via code_checks?

[rhshadrach]

I should have googled first... pre-commit run --all-files works great.