messageTray: don't force a size on SourceActor's iconBin

Supports hidpi icons this way.

https://bugzilla.gnome.org/show_bug.cgi?id=724607

js/ui/messageTray.js

@@ -1162,9 +1162,7 @@ const SourceActor = new Lang.Class({
         }));
         this._actorDestroyed = false;
 
-        this._iconBin = new St.Bin({ width: size,
-                                     height: size,
-                                     x_fill: true,
+        this._iconBin = new St.Bin({ x_fill: true,
                                      y_fill: true });
 
         this.actor.add_actor(this._iconBin);