magnifier: Stop exposing D-Bus interface
Screen magnification is the compositor's business, not that of "random" unprivileged tools. And for cases where a more specialised behavior is wanted, an extension likely does a better job than a consumer of the D-Bus API.
In addition to that, exporting the interface has been broken for an unknown time, because the object that holds the implementation isn't referenced and thus ends up being garbage collected, whoops.
And last but not least, this gets rid of the last public D-Bus name that isn't clearly in the system namespace (org.gnome.Shell, org.gnome.Mutter, org.gtk).