mptcp: Remove unused inline function mptcp_rcv_synsent()

commit 263e1201 ("mptcp: consolidate synack processing.")
left behind this, remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>