gh-129185: Simplify PyTraceMalloc_Track() (#129256)

Since tracemalloc uses PyMutex, it becomes safe to use TABLES_LOCK()
even after _PyTraceMalloc_Fini(): remove the "pre-check" in
PyTraceMalloc_Track() and PyTraceMalloc_Untrack().

PyTraceMalloc_Untrack() no longer needs to acquire the GIL.

_PyTraceMalloc_Fini() can be called earlier during Python
finalization.