ENH: Remove redundant vtkImageAccumulateDiscrete already provided by VTK

Using Slicer implementation of the ImageAccumulate filter is equivalent
to the VTK implementation after setting ComponentExtent and ComponentOrigin
appropriately to account for signed 16-bit images.

Output of the filter have been verified for both MRHead and ChestCT
sample data.

Associated discussion: http://slicer-devel.65872.n3.nabble.com/Slicer-vtkImageAccumulateDiscrete-Should-we-just-use-the-class-provided-by-VTK-tp4030447p4030451.html

See #3485

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22768 3bd1e089-480b-0410-8dfb-8563597acbee