mptcp: drop useless ssk in pm_subflow_check_next

mptcp: drop useless ssk in pm_subflow_check_next

The code using 'ssk' parameter of mptcp_pm_subflow_check_next() has been
dropped in commit "95d686517884 (mptcp: fix subflow accounting on close)".
So drop this useless parameter ssk.

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231025-send-net-next-20231025-v1-4-db8f25f798eb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>