Remove NMDefaultRouteManager. Instead, add the default-route to the
NMIP4Config/NMIP6Config instance.

This basically reverts commit e8824f6a.
We added NMDefaultRouteManager because we used the corresponding to `ip
route replace` when configuring routes. That would replace default-routes
on other interfaces so we needed a central manager to coordinate routes.
Now, we use the corresponding of `ip route append` to configure routes,
and each interface can configure routes indepdentently.

In NMDevice, when creating the default-route, ignore @auto_method for
external devices. We shall not touch these devices.

Especially the code in NMPolicy regarding selection of the best-device
seems wrong. It probably needs further adjustments in the future.
Especially get_best_ip_config() should be replaced, because this
distinction VPN vs. devices seems wrong to me.
Thereby, remove the @ignore_never_default argument. It was added by
commit bb750260, I don't think it's
needed anymore.

This brings another change. Now that we track default-routes in
NMIP4Config/NMIP6Config, they are also exposed on D-Bus like regular
routes. I think that makes sense, but it is a change in behavior, as
previously such routes were not exposed there.