COMP: Fix "-Wunused-but-set-variable" in vtkMRMLModelDisplayableManager

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22558 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Fix [-Wunused-but-set-variable] in vtkSlicerVolumeRenderingLogic

After checking with the author of the class, it makes sense to remove the
code related to the unused "gradientRange" variable.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22559 3bd1e089-480b-0410-8dfb-8563597acbee