sctp: remove completely unsed EMPTY state

SCTP does not SCTP_STATE_EMPTY and we can never be in
that state.  Remove useless code.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>