Removed the using to avoid VS 7.1 build problem

include/osg/ClusterCullingCallback

@@ -54,10 +54,6 @@ class OSG_EXPORT ClusterCullingCallback : public Drawable::CullCallback, public
         void setDeviation(float deviation) { _deviation = deviation; }
         float getDeviation() const { return _deviation; }
 
-        /* Not all virtual overloads of drawImplementation are overridden in this class, so
-           bring the base class implementation in to avoid hiding the non-used ones. */
-        using Drawable::CullCallback::cull;
-
         virtual bool cull(osg::NodeVisitor*, osg::Drawable*, osg::State*) const;
 
         /** Callback method called by the NodeVisitor when visiting a node.*/