Let's not treat those routes special. I think this was originally done, because
we relied on kernel to add the IPv4 device route, so we would ignore RTPROT_KERNEL
routes and not delete them.

We want to track them for various reasons:

 - for consistency, there is nothing special except that they might be
   added by kernel.
 - we expose the routes of NMIP4Config/NMIP6Config on D-Bus. That should
   include also routes such as device routes. Note, this commit changes
   that we now expose device routes on D-Bus too.