BUG: remove extra node added event
requested to merge github/fork/naucoin/4033-Duplicated-event-when-a-new-vtkMRMLScalarVolumeNode-is-added-to-the-scene into master
When switching to using a mini scene to try various ways of loading volumes, the final node added event wasn't removed when I switched to using AddNode to move the final set of nodes into the main scene. This change removes the extra node added event triggered by adding a volume.
Issue #4033