target: Remove unused su_group usage in fabric register/dergister

This patch removes two instances of left over v3.x code performing local
scope access to struct target_core_fabric_ops->tf_subsys->su_group in
target_fabric_configfs_register() and target_fabric_configfs_deregister().

Reported-by: Christophe Fergeau <cfergeau@gmail.com>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>