unlockDialog: Don't explicitly show prompt

The promptBox is initially fully opaque, so showing it before the
transition can result in a brief flash before fading in.

Just remove the show() call and let the transition handle the
visibility.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2212

js/ui/unlockDialog.js

@@ -665,7 +665,6 @@ var UnlockDialog = GObject.registerClass({
             return;
 
         this._activePage = this._promptBox;
-        this._promptBox.show();
 
         this._adjustment.ease(1, {
             duration: CROSSFADE_TIME,