stop retrieving now-unscriptable nsIDOMGeoGeolocation interface from geolocation component

[Bug 850442](https://bugzilla.mozilla.org/show_bug.cgi?id=850442) in [part 4 of its changes](http://hg.mozilla.org/mozilla-central/rev/23e907c7c106) made nsIDOMGeoGeolocation nonscriptable, but we can still access the component (and that interface, strangely) by requesting its nsISupports interface instead.