gnome3.glib-networking: replace with alias

Since we moved gsettings-desktop-schemas to top-level, gnome3.glib-networking was the same as glib-networking.
We could try to make the top-level variant not depend on gsettings-desktop-schemas again but that is probably
pointless, as the dependency is rather small compared to things like libproxy. Instead, we will just drop
the package in gnome3 attr set and always rely on the top-level expression.