LibGUI: Remove ancient code for debugging widget underdraw

Libraries/LibGUI/Widget.cpp

@@ -225,13 +225,6 @@ void Widget::handle_paint_event(PaintEvent& event)
     if (fill_with_background_color()) {
         Painter painter(*this);
         painter.fill_rect(event.rect(), palette().color(background_role()));
-    } else {
-#ifdef DEBUG_WIDGET_UNDERDRAW
-        // FIXME: This is a bit broken.
-        // If the widget is not opaque, let's not mess it up with debugging color.
-        Painter painter(*this);
-        painter.fill_rect(rect(), Color::Red);
-#endif
     }
     paint_event(event);
     for_each_child_widget([&](auto& child) {