sctp: Remove unused declaration sctp_backlog_migrate()

Commit 61c9fed41638 ("[SCTP]: A better solution to fix the race between sctp_peeloff()
and sctp_rcv().") removed the implementation but left declaration in place. Remove it.

Signed-off-by: Yue Haibing <[e-mail]>
Acked-by: Xin Long <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/sctp/sctp.h

···
 void sctp_icmp_proto_unreachable(struct sock *sk,
 				 struct sctp_association *asoc,
 				 struct sctp_transport *t);
-void sctp_backlog_migrate(struct sctp_association *assoc,
-			  struct sock *oldsk, struct sock *newsk);
 int sctp_transport_hashtable_init(void);
 void sctp_transport_hashtable_destroy(void);
 int sctp_hash_transport(struct sctp_transport *t);