CI move code directives to pre-commit, remove some outdated checks

[MarcoGorelli]

Moving checks for incorrect code block or IPython directives to pre-commit so they're cross-platform and give faster feedback to devs.

Also removing some no-longer-necessary code checks:

• "Check that no file in the repo contains trailing whitespaces" is taken care of by the trailing-whitespace hook
• "Check for extra blank lines after the class definition" is enforced by black

[jbrockmendel]

LGTM pending green

[jreback]

thanks @MarcoGorelli