LayoutManager: Remove broken startup animation

It doesn't make sense to animate blindly a MetaBackgroundActor, given that
it shows the content of _XROOTPMAP_ID, so if gnome-settings-daemon is fast
we're animating the configured background, not the plymouth screen. And anyway
it would be animated on top of the standard MetaBackgroundActor...
It makes even less sense now that mutter renders the background on its own
(and blocks the first paint cycle until the background image is ready)
We need to do something better here, but for now, remove this.

https://bugzilla.gnome.org/show_bug.cgi?id=688210

js/ui/layout.js

@@ -111,7 +111,6 @@ const LayoutManager = new Lang.Class({
         this.primaryIndex = -1;
         this._keyboardIndex = -1;
         this._hotCorners = [];
-        this._background = null;
         this._leftPanelBarrier = 0;
         this._rightPanelBarrier = 0;
 
@@ -379,38 +378,8 @@ const LayoutManager = new Lang.Class({
     _startupAnimation: function() {
         this.panelBox.translation_y = -this.panelBox.height;
 
-        let plymouthTransitionRunning = false;
-
-        // If we're the greeter, put up the xrootpmap actor
-        // and fade it out to have a nice transition from plymouth
-        // to the greeter. Otherwise, we'll just animate the panel,
-        // as usual.
-        if (Main.sessionMode.isGreeter) {
-            this._background = Meta.BackgroundActor.new_for_screen(global.screen);
-            if (this._background != null) {
-                Main.uiGroup.add_actor(this._background);
-                Tweener.addTween(this._background,
-                                 { opacity: 0,
-                                   time: PLYMOUTH_TRANSITION_TIME,
-                                   transition: 'linear',
-                                   onComplete: this._fadeBackgroundComplete,
-                                   onCompleteScope: this });
-                plymouthTransitionRunning = true;
-            }
-        }
-
-        if (!plymouthTransitionRunning)
-            this._fadeBackgroundComplete();
-    },
-
-    _fadeBackgroundComplete: function() {
         this._freezeUpdateRegions();
 
-        if (this._background != null) {
-            this._background.destroy();
-            this._background = null;
-        }
-
         Tweener.addTween(this.panelBox,
                          { translation_y: 0,
                            time: STARTUP_ANIMATION_TIME,