iscsi-target: Remove no-op from iscsit_tpg_del_portal_group

This patch removes a no-op iscsit_clear_tpg_np_login_threads() call
in iscsit_tpg_del_portal_group(), which is unnecessary because
iscsit_tpg_del_portal_group() can only ever be removed from configfs
once all of the child network portals have been released.

Also, go ahed and make iscsit_clear_tpg_np_login_threads() declared
as static.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>