gh-152238: Revert gh-150490 and gh-152200. (gh-152232)

gh-152238: Revert gh-150490 and gh-152200. (gh-152232)

Revert commits:
    gh-151593: Fix dead lock in PyDict insert_split_key() (#152200)
    gh-150490: Raise PyType_Modified for insertion into split dictionary (#150489)

For gh-150489, it violates locking discipline and results in deadlocks,
gh-151593 is an example of it being hit in CI.  The attempted fix
gh-152200 avoids the deadlock but introduces a data-race.  The race
window is small but can be triggered with pure Python code.