sctp: drop the old assoc hashtable of sctp

transport hashtable will replace the association hashtable,
so association hashtable is not used in sctp any more, so
drop the codes about that.

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