sctp: Remove unused sctp_assoc_del_peer and sctp_chunk_iif

sctp: Remove unused sctp_assoc_del_peer and sctp_chunk_iif

sctp_assoc_del_peer() last use was removed in 2015 by
commit 73e6742027f5 ("sctp: Do not try to search for the transport twice")
which now uses rm_peer instead of del_peer.

sctp_chunk_iif() last use was removed in 2016 by
commit 1f45f78f8e51 ("sctp: allow GSO frags to access the chunk too")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20250501233815.99832-1-linux@treblig.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>