LibGUI: Make GUI::Frame have the 2px sunken container look by default

The overwhelming majority of GUI::Frame users set the same appearance,
so let's just make it the default.

Libraries/LibHTML/HtmlView.cpp

@@ -59,9 +59,6 @@ HtmlView::HtmlView()
         update(adjusted_rect);
     };
 
-    set_frame_shape(Gfx::FrameShape::Container);
-    set_frame_shadow(Gfx::FrameShadow::Sunken);
-    set_frame_thickness(2);
     set_should_hide_unnecessary_scrollbars(true);
     set_background_role(ColorRole::Base);
 }