netdevsim: Allow programming routes with nexthop objects

Previous patches added the ability to program nexthop objects.
Therefore, no longer forbid the programming of routes that point to such
objects.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>