Clear render pakcage for node that moved to custom workspace

This pull request is to fix defect [MAGN 5165: Custom node not cleaning up its old node output](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5165).

That is because each node has ``RenderPackage`` which will be used by ``VisualizationManager`` to do geometry preview. When nodes are converted to custom node, their ``RenderPackage`` won't be updated anymore because technically all these nodes are out-of-scope when the execution returns from the function, therefore no ``RenderPackage`` will be generated for them, and in this case ``VisualizationManager`` uses old-dated ``RenderPackage`` to do preview. We should clear ``RenderPackage`` when moving a node to custom work space. 

@lukechurch  for your review.