When calling update_default_route(), NMDefaultRouteManager will look at the
source, and determine whether it has a default route or not. For example
for device sources, this means calling nm_device_get_ip4_default_route().

If the source indicates that it has no default route, the effect of
calling update_default_route() is the same as calling
remove_default_route() (hence, remove() can be replaced by update()).

If the source however still indicates a default route, the behavior
would be different. This case would be an undesired inconsistancy,
because source and NMDefaultRouteManager would disagree of whether
the source has a default route.
Source must always properly indicate whether it has a default route
or not, hence this situation does not arise.

Hence it is always better to call update().

Signed-off-by: Thomas Haller <thaller@redhat.com>