mlxsw: spectrum_router: Allow programming routes with nexthop objects

Now that the driver supports nexthop objects, the check is no longer
necessary. Remove it.

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