selftests: mptcp: remove rp_filter configuration

Remove the rp_filter configuration from MPTCP tests, as it is now handled
by setup_ns.

Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250508081910.84216-7-liuhangbin@gmail.com


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