gh-105716: Fix _PyInterpreterState_IsRunningMain() For Embedders (gh-117140)

gh-105716: Fix _PyInterpreterState_IsRunningMain() For Embedders (gh-117140)

When I added _PyInterpreterState_IsRunningMain() and friends last year, I tried to accommodate applications that embed Python but don't call _PyInterpreterState_SetRunningMain() (not that they're expected to).  That mostly worked fine until my recent changes in gh-117049, where the subtleties with the fallback code led to failures; the change ended up breaking test_tools.test_freeze, which exercises a basic embedding situation.

The simplest fix is to drop the fallback code I originally added to _PyInterpreterState_IsRunningMain() (and later to _PyThreadState_IsRunningMain()).  I've kept the fallback in the _xxsubinterpreters module though.  I've also updated Py_FrozenMain() to call _PyInterpreterState_SetRunningMain().