mptcp: drop sndr_key in mptcp_syn_options

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: Geliang Tang <geliangtang@gmail.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>