cpython

(cherry picked from commit caa279d6)

This was added for bpo-40514 (gh-84694) to test out a per-interpreter GIL.  However, it has since proven unnecessary to keep the experiment in the repo.  (It can be done as a branch in a fork like normal.)  So here we are removing:

* the configure option
* the macro
* the code enabled by the macro

Automerge-Triggered-By: GH:ericsnowcurrently