mptcp: drop unused sk in mptcp_push_release

Since mptcp_set_timeout() had removed from mptcp_push_release() in
commit 33d41c9c ("mptcp: more accurate timeout"), the argument
sk in mptcp_push_release() became useless. Let's drop it.

Fixes: 33d41c9c ("mptcp: more accurate timeout")
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>