QmlProject: Clean tree before re-population

Regression after 9e8b427a70

Change-Id: I34c96e1e8f14c6fc76e744f9847060a3094bb005
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/qmlprojectmanager/qmlprojectnodes.cpp

@@ -73,6 +73,7 @@ void QmlProjectNode::refresh()
     });
     fileNodes.append(projectFilesNode);
 
+    makeEmpty();
     buildTree(fileNodes);
 }