STYLE: vtkMRMLSegmentationNode: Fix unused-parameter segmentId warnings

This commit fixes these warnings:

/path/to/Slicer/Libs/MRML/Core/vtkMRMLSegmentationNode.cxx:265:58: warning: unused parameter ‘segmentId’ [-Wunused-parameter]
void vtkMRMLSegmentationNode::OnSegmentAdded(const char segmentId)
^
/path/to/Slicer/Libs/MRML/Core/vtkMRMLSegmentationNode.cxx:270:60: warning: unused parameter ‘segmentId’ [-Wunused-parameter]
void vtkMRMLSegmentationNode::OnSegmentRemoved(const char segmentId)
^
/path/to/Slicer/Libs/MRML/Core/vtkMRMLSegmentationNode.cxx:275:61: warning: unused parameter ‘segmentId’ [-Wunused-parameter]
void vtkMRMLSegmentationNode::OnSegmentModified(const char* segmentId)
^

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25599 3bd1e089-480b-0410-8dfb-8563597acbee