selftests: netfilter: remove rp_filter configuration

Remove the rp_filter configuration in netfilter lib, as setup_ns already
sets it appropriately by default

Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250508081910.84216-6-liuhangbin@gmail.com


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