Remove legacy env.var in QQuickWidget comparison example

Seems to work regardless.

Pick-to: 6.5
Change-Id: Ic7c58372181009214590e9152a4919234f8e4aaa
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

examples/quick/quickwidgets/qquickwidgetversuswindow_opengl/main.cpp

@@ -9,8 +9,6 @@
 
 int main(int argc, char **argv)
 {
-    qputenv("QML_BAD_GUI_RENDER_LOOP", "1"); // QTBUG-39507
-
     QApplication app(argc, argv);
 
     // this example and QQuickWidget are only functional when rendering with OpenGL