Removed dead code found by -Wunused-template
Fixes dashboard warning:
VTK/Rendering/OpenGL2/vtkDualDepthPeelingPass.cxx:880:28: warning: unused function template 'DeleteHelper' [-Wunused-template]
Fixes dashboard warning:
VTK/Rendering/OpenGL2/vtkDualDepthPeelingPass.cxx:880:28: warning: unused function template 'DeleteHelper' [-Wunused-template]