mptcp: remove unused returned value of check_data_fin

mptcp: remove unused returned value of check_data_fin

When working on a fix modifying mptcp_check_data_fin(), I noticed the
returned value was no longer used.

It looks like it was used for 3 days, between commit 7ed90803a213
("mptcp: send explicit ack on delayed ack_seq incr") and commit
ea4ca586b16f ("mptcp: refine MPTCP-level ack scheduling").

This returned value can be safely removed.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250919-net-next-mptcp-server-side-flag-v1-6-a97a5d561a8b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>