[3.10] bpo-46006: Revert "bpo-40521: Per-interpreter interned strings (GH-20085)" (GH-30422) (GH-30425)

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.

Leave the PyInterpreterState structure unchanged to keep the ABI
backward compatibility with Python 3.10.0: rename the "interned"
member to "unused_interned".

(cherry picked from commit 35d6540c904ef07b8602ff014e520603f84b5886)