selftests: mptcp: connect: remove unused variable

'cin_disconnect' is used in run_tests_disconnect(), but not
'cout_disconnect', so it is safe to drop it.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250114-net-next-mptcp-st-more-debug-err-v1-5-2ffb16a6cf35@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>