sctp: remove the unused sctp_assoc_is_match function

After Commit 4f008781 ("sctp: apply rhashtable api to send/recv
path"), there's no place using sctp_assoc_is_match, so remove it.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>