Remove python integration test in-the-past scaffolding

[aarongable]

Delete test_ocsp_resigning and test_ocsp_exp_unauth. Although these tests are not replicated in our Go integration tests, they are testing very stable parts of our code that we do not plan to modify until we delete it entirely.

Removing these tests allows us to remove the "setup_six_months_ago" and "setup_twenty_days_ago" scaffolding in our python integration test harness, greatly simplifying that python code and reducing integration test runtime by about 30 seconds.

Part of #4934

[jsha]

I like the idea of saving 30 seconds on our integration tests, and I agree that this code is stable and going away soon with the removal of OCSP services. But we shouldn't land this before the actual removal of OCSP services, since we are still responsible for correct operation until then. Let's hold this PR until that happens. Other than the question of timing, looks good.

Also lol that setup_six_months_ago() uses a list comprehension and setup_twenty_days_ago() uses a for loop even though they do the exact same thing and are very close to each other. :D