Remove MULE_INTERNAL encoding.

Remove MULE_INTERNAL encoding.

This was useful before widespread Unicode adoption, and was based on the
internal encoding Emacs used to mix multiple sub-encodings.  Emacs
itself has stopped using it, and our implementation hadn't been updated
with modern underlying standards.  It is thought to be very unlikely
that anyone is still using it in the field.  Since such a complex
encoding comes with costs and risks, we agreed to drop support.

Any existing database using this encoding would need to be dumped and
restored with a new encoding to upgrade to PostgreSQL 19, most likely
UTF8, since pg_upgrade would fail.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Reviewed-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/CA%2BhUKGKXDXh-FdU0orjfv%2BF08f%3DD91BhV3Ra-4zL-q%2BJmGYqTA%40mail.gmail.com