Gui: Cleanup Qt OpenGL usings in QtOpenGL.h.

Previously the code defined compatiblity usings in `QtOpenGL.h` header,
which I think was added for backwards compatiblity with previous Qt
OpenGL widgets.

As far as I can tell, this is not necessary anymore, and can be cleaned
up.

src/Gui/SoTextLabel.cpp

@@ -307,7 +307,7 @@ SoStringLabel::SoStringLabel()
  */
 void SoStringLabel::GLRender(SoGLRenderAction *action)
 {
-    QtGLWidget* window;
+    QOpenGLWidget* window;
     SoState * state = action->getState();
     state->push();
     SoLazyElement::setLightModel(state, SoLazyElement::BASE_COLOR);