aura-mus: Remove dead code.

ui/views/widget/widget.h

Index: ui/views/widget/widget.h
diff --git a/ui/views/widget/widget.h b/ui/views/widget/widget.h
index 505beb1e9e4b3b5d9c4b1fc3091230c61326a26a..2a239c43013a9755decbf4cf3b49086f05333a55 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -57,7 +57,6 @@ class Layer;
 class NativeTheme;
 class OSExchangeData;
 class ThemeProvider;
-class Window;
 }  // namespace ui
 
 namespace wm {
@@ -254,8 +253,6 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
     // Whether the widget should be maximized or minimized.
     ui::WindowShowState show_state;
     gfx::NativeView parent;
-    // Used only by mus and is necessitated by mus not being a NativeView.
-    ui::Window* parent_mus = nullptr;
     // Specifies the initial bounds of the Widget. Default is empty, which means
     // the NativeWidget may specify a default size. If the parent is specified,
     // |bounds| is in the parent's coordinate system. If the parent is not