gh-117657: Remove remaining TSan suppressions

[colesbury]

• Issue: Make TSAN tests pass with the GIL disabled in free-threaded builds #117657

[colesbury]

Thread leaks were fixed (or skipped) in:

• gh-143121: Avoid thread leak in configure #143122
• gh-143121: Skip test that leak threads under TSan #143125

I'm not sure when we fixed _PyObject_TryGetInstanceAttribute(), but I've been running the test suite repeatedly locally without suppressions and haven't see any races reported for it.

[Yhg1s]

Let's add a "this page intentionally left blank" comment, to make it clear this really is meant to be empty but we don't want to delete it because tooling might be expect it to exist in 3.15 like it did in 3.14. (I assume that's why you aren't deleting it.)