COMP: Remove unused vtkSlicerMesaRayCastImageDisplayHelper classes

This class first introduced by commit r4673 (ENH: Integrated VTK ray
cast classes, started with modifications for incremental update) has been
copied in the code based but was not compiled.

Then, none of following commit (See [1][2][3]) related to this class
updated the CMakeLists.txt to compile the file.

This commit takes care of removing this unused files.

[1] r5517: ENH: Create own dll for vtkReplacements
[2] r9952: ENH: added new volume rendering module
[3] r15837: ENH: added first cut of VolumeRendering module

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23918 3bd1e089-480b-0410-8dfb-8563597acbee