COMP: Remove unused class "Base/Logic/vtkSlicerIGTDemoLogic"

This class has originally been added with Slicer r1840  (back in 2006) and
was first used by vtkSlicerIGTDemoGUI added at r1843. Then, at r4954 the
GUI classes have been removed while keeping around the Logic.

Afterward, considering that (1) logic class hasn't been used anywhere
and (2) wasn't build. It's is safe to remove this class.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21739 3bd1e089-480b-0410-8dfb-8563597acbee