COMP: Remove unused ivar from vtkMRMLScalarVolumeNode

Following commit r17798, calculation of [Scalar]AutoLevels has been
moved into vtkMRMLScalarVolumeDisplayNode. Therefore, the file has been
updated.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22766 3bd1e089-480b-0410-8dfb-8563597acbee