Remove unreferenced #include lines

Fixes compile errors when building against a VTK install tree
Allow compilation with VTK 5.8.

The use of vtkImageHistogramStatistics kept the programs from
compiling with VTK 5.8.  Since the role of this filter is minor,
I made its use depend on the VTK version.  Unfortunately, the
image display examples had to be disabled completely on VTK 5.8.