COMP: Fix unused-but-set-variable warning in vtkImageErode

This commit fixes the following warning happening when building Slicer
in Release:

/path/to/Slicer/Modules/Scripted/EditorLib/Logic/vtkImageErode.cxx: In instantiation of ‘void vtkImageErodeExecute(vtkImageErode, vtkImageData, T, vtkImageData, int*, int) [with T = double]’:
/path/to/Slicer/Modules/Scripted/EditorLib/Logic/vtkImageErode.cxx:217:26: required from here
/path/to/Slicer/Modules/Scripted/EditorLib/Logic/vtkImageErode.cxx:76:25: warning: variable ‘tDiff’ set but not used [-Wunused-but-set-variable]
clock_t tStart, tEnd, tDiff;

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23603 3bd1e089-480b-0410-8dfb-8563597acbee