removes unused property from HeaderController

Author: velesin

app/assets/javascripts/discourse/controllers/header_controller.js

@@ -19,10 +19,6 @@ Discourse.HeaderController = Discourse.Controller.extend({
     return Discourse.User.current() && !this.get('topic.isPrivateMessage');
   }.property('topic.isPrivateMessage'),
 
-  mobileDevice: function() {
-    return Discourse.Mobile.isMobileDevice;
-  }.property(),
-
   mobileView: function() {
     return Discourse.Mobile.mobileView;
   }.property(),