bpo-46006: Revert "bpo-40521: Per-interpreter interned strings (GH-20085)" (GH-30422)

This reverts commit ea251806b8dffff11b30d2182af1e589caf88acf.

Keep "assert(interned == NULL);" in _PyUnicode_Fini(), but only for
the main interpreter.

Keep _PyUnicode_ClearInterned() changes avoiding the creation of a
temporary Python list object.