mptcp: sched: remove mptcp_sched_data

mptcp: sched: remove mptcp_sched_data

This is a follow-up of commit b68b106b0f15 ("mptcp: sched: reduce size
for unused data"), now removing the mptcp_sched_data structure.

Now is a good time to do that, because the previously mentioned WIP work
has been updated, no longer depending on this structure.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250413-net-next-mptcp-sched-mib-sft-misc-v2-1-0f83a4350150@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>