gh-116522: Refactor `_PyThreadState_DeleteExcept` (#117131)

Split `_PyThreadState_DeleteExcept` into two functions:

- `_PyThreadState_RemoveExcept` removes all thread states other than one
  passed as an argument. It returns the removed thread states as a
  linked list.

- `_PyThreadState_DeleteList` deletes those dead thread states. It may
  call destructors, so we want to "start the world" before calling
  `_PyThreadState_DeleteList` to avoid potential deadlocks.