Dropping elemental-forms and swapping out with imgui.

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

src/xenia/gpu/trace_viewer.h

@@ -21,7 +21,6 @@
 
 namespace xe {
 namespace ui {
-class ImGuiDrawer;
 class Loop;
 class Window;
 }  // namespace ui
@@ -62,8 +61,6 @@ class TraceViewer {
   GraphicsSystem* graphics_system_ = nullptr;
   std::unique_ptr<TracePlayer> player_;
 
-  std::unique_ptr<xe::ui::ImGuiDrawer> imgui_drawer_;
-
  private:
   enum class ShaderDisplayType : int {
     kUcode,