Do not use partially checked state of legend nodes when layer is not visible

This is slightly confusing, moreover in some styles the items are not immediately
updated, producing a weird effect when hovering mouse over legend nodes.
http://www.youtube.com/watch?v=HNDE1Xp5UZA&feature=youtu.be

Author: wonder-sk

src/core/layertree/qgslayertreemodellegendnode.cpp

@@ -164,9 +164,6 @@ QVariant QgsSymbolV2LegendNode::data( int role ) const
     if ( !mItem.isCheckable() )
       return QVariant();
 
-    if ( !mLayerNode->isVisible() )
-      return Qt::PartiallyChecked;
-
     QgsVectorLayer* vlayer = qobject_cast<QgsVectorLayer*>( mLayerNode->layer() );
     if ( !vlayer || !vlayer->rendererV2() )
       return QVariant();