Remove WITH_FREELISTS macro #123990

As we discussed at https://discuss.python.org/t/should-we-remove-the-with-freelists-macro,
we just added this macro to test mimalloc's own feature without using our own free list implementation.
See: #89685

But since we already implemented per thread freelist at the #111968 for freethreading, we don't need this flag anymore.

I will remove this macro during this year's core sprint (only around two weeks left)
https://discuss.python.org/t/2024-core-dev-sprint-in-bellevue-wa-september-23-27/39227

cc @colesbury @vstinner @markshannon

Linked PRs

• gh-123990: Good bye WITH_FREELISTS macro #124358