In RFC 8684, we don't need to send sndr_key in SYN package anymore, so drop
it.

Fixes: cc7972ea ("mptcp: parse and emit MP_CAPABLE option according to v1 spec")
Signed-off-by: NGeliang Tang <geliangtang@gmail.com>
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>