res_pjsip_outbound_registration.c: Use ast_sorcery_object_unregister() API

The sorcery pjsip 'registration' config object needs to be destroyed on
module unload.  Otherwise, a reload of res_pjsip could try to use
callbacks for a previously unloaded instance of the module provided by
ast_sorcery_object_register() or one of the variants.  Also, if
res_pjsip_outbound_registration were subsequently reloaded, the sorcery
config field objects would be registered in sorcery twice.

ASTERISK-24907
Reported by: Kevin Harwell

Change-Id: I304fad13dece2604af48353f6c6d9d5c7b064697