auth3: Remove auth_skel.c

auth3: Remove auth_skel.c

Authentication is a very complex topic, and someone who is able to
write a custom auth module turning a struct auth_usersupplied_info
into a struct auth_serversupplied_info should be able to live without
this skeleton module.

This module also gave an example to load a secondary authentication
module via a module parameter (the call to load_module()). We have
abandoned this practice, and since the "auth methods" parameter has
gone we don't use this anymore internally.

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