sctp: Remove unused declaration sctp_backlog_migrate()

Commit 61c9fed4

 ("[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 <yuehaibing@huawei.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20230809142323.9428-1-yuehaibing@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>