smbd4: Remove process_model_thread

smbd4: Remove process_model_thread

This code has not seen any use or development for a while. While probably
being a good source of inspiration, it will inevitably bit-rot. To really
get to a multi-threaded process model, many of the Samba libraries have
to be made thread-safe.

Why this patch? Right now I am trying to clean up our debug system,
and process_thread.c is the only reference to the _suspicious_ functions
in debug.c. The alternative would be to factor those out of debug, but
as this code is unused for so long, the alternative approach is to just
remove it.

If someone wants to restart development on a threaded process model,
the code is still around in the git history and can be resurrected easily.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>