This property is currently most likely not used. Also, because libnm doesn't
expose it and the only known user of this API (gnome-network-displays) doesn't
use it.

In the future we may want to expand on the Groups API. E.g. exposing groups as
separate D-Bus objects, in which case a better property type would be "ao" and
not "as". For now, that is unclear nor requested.

Remove the property for now.