Fix build against OSG 3.5.3

extern/osgQt/GraphicsWindowQt

@@ -37,9 +37,6 @@ namespace osgQt
 // forward declarations
 class GraphicsWindowQt;
 
-/// The function sets the WindowingSystem to Qt.
-void initQtWindowingSystem();
-
 /** The function sets the viewer that will be used after entering
  *  the Qt main loop (QCoreApplication::exec()).
  *

extern/osgQt/GraphicsWindowQt.cpp

@@ -639,20 +639,6 @@ private:
 };
 
 
-// declare C entry point for static compilation.
-extern "C" void graphicswindow_Qt(void)
-{
-    osg::GraphicsContext::setWindowingSystemInterface(QtWindowingSystem::getInterface());
-}
-
-
-void osgQt::initQtWindowingSystem()
-{
-    graphicswindow_Qt();
-}
-
-
-
 void osgQt::setViewer( osgViewer::ViewerBase *viewer )
 {
     HeartBeat::instance()->init( viewer );