selftests: forwarding: Do not configure nexthop objects twice

routing_nh_obj() is used to configure the nexthop objects employed by
the test, but it is called twice resulting in "RTNETLINK answers: File
exists" messages.

Remove the first call, so that the function is only called after
setup_wait(), when all the interfaces are up and ready.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>