COMP: vtkMRMLCropVolumeParametersNode: Fix -Wunused-variable

This commit fixes the following warning:

/path/to/Slicer/Modules/Loadable/CropVolume/MRML/vtkMRMLCropVolumeParametersNode.cxx:35:20: warning: unused variable 'ROIAlignmentTransformNodeReferenceMRMLAttributeName' [-Wunused-variable]
static const char* ROIAlignmentTransformNodeReferenceMRMLAttributeName = "ROIAlignmentTransformNodeID";
^

From Andras: the unused ROIAlignmentTransformNodeReferenceMRMLAttributeName
can be removed (other node references have custom node attributes defined
for backward compatibility with old scenes when node references were not
supported yet).

Reviewed-by: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25836 3bd1e089-480b-0410-8dfb-8563597acbee