Dropping elemental-forms and swapping out with imgui.

Too much code for such little use. This should simplify porting.

src/xenia/ui/loop.h

@@ -23,11 +23,6 @@ class Loop {
  public:
   static std::unique_ptr<Loop> Create();
 
-  static Loop* elemental_loop();
-  // Sets the loop designated as being the main loop elemental-forms will use
-  // for all activity.
-  static void set_elemental_loop(Loop* loop);
-
   Loop();
   virtual ~Loop();