[3.11] bpo-40514: Drop EXPERIMENTAL_ISOLATED_SUBINTERPRETERS (gh-93185) (GH-93306)

(cherry picked from commit caa279d6fd5f151e57f891cd4f6ba51b532501c6)

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