ENH: Remove ITKv3 support

As discussed on the mailing list [1] and reported on the Roadmap [2],
this commit removes all conditional code that was allowing Slicer4 to be
compiled against ITKv3.

Code that was building only against ITKv3 has also been removed. This
includes DicomToNrrdConverter CLI module
and SlicerITKv3N4MRIBiasFieldCorrectionImageFilter class.

[1] http://slicer-devel.65872.n3.nabble.com/Removal-of-ITKv3-support-tt4031364.html

[2] http://wiki.slicer.org/slicerWiki/index.php/Roadmap#Deprecation_.2F_API_Changes

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23077 3bd1e089-480b-0410-8dfb-8563597acbee