dash: Make sure item labels are only destroyed once

Labels are currently destroyed from both animateOutAndDestroy()
and destroy(), which now (rightfully) triggers a gjs warning. As
the label is created unconditionally since commit 36e5ae4a25,
mirror that and always release it in destroy() and hide it
elsewhere.

https://bugzilla.gnome.org/show_bug.cgi?id=791233

js/ui/dash.js

@@ -178,15 +178,12 @@ var DashItemContainer = new Lang.Class({
     },
 
     destroy: function() {
-        if (this.label)
         this.label.destroy();
-
         this.parent();
     },
 
     animateOutAndDestroy: function() {
-        if (this.label)
-            this.label.destroy();
+        this.label.hide();
 
         if (this.child == null) {
             this.destroy();