sctp: Do not try to search for the transport twice

When removing an non-primary transport during ASCONF
processing, we end up traversing the transport list
twice: once in sctp_cmd_del_non_primary, and once in
sctp_assoc_del_peer.  We can avoid the second
search and call sctp_assoc_rm_peer() instead.
Found by code inspection during code reviews.

Signed-off-by: Vladislav Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>