[3.14] gh-135450: Remove assertion in `_PyCode_CheckNoExternalState` (gh-135694)

The assertion reflected a misunderstanding of situations where "hidden" variables might exist,
namely generator expressions and comprehensions.

(cherry picked from commit 15f2bac02c5e, AKA gh-135466)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>