Removed unused function sctp_addr_is_valid()

sctp_addr_is_valid() only appeared in its definition.

Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Sébastien Barré <sebastien.barre@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>