gh-85858: Remove PyUnicode_InternImmortal() function (#92579)

Remove the PyUnicode_InternImmortal() function and the
SSTATE_INTERNED_IMMORTAL macro.

The PyUnicode_InternImmortal() function is still exported in the
stable ABI. The function is removed from the API.

PyASCIIObject.state.interned size is now a single bit, rather than 2
bits.

Keep SSTATE_NOT_INTERNED and SSTATE_INTERNED_MORTAL macros for
backward compatibility, but no longer use them internally since the
interned member is now a single bit and so can only have two values
(interned or not interned).

Update stats of _PyUnicode_ClearInterned().