Fixed all -Wunused-member-function warnings, mostly by removing dead code

Common/Core/Testing/Cxx/TestDataArrayIterators.cxx

Common/DataModel/Testing/Cxx/TestMappedGridDeepCopy.cxx

Common/DataModel/vtkStaticPointLocator2D.cxx

...xecutionModel/Testing/Cxx/TestMultiOutputSimpleFilter.cxx

Filters/Core/vtkPlaneCutter.cxx

Filters/Core/vtkVoronoi2D.cxx

Filters/Points/vtkPointDensityFilter.cxx

IO/Geometry/vtkHoudiniPolyDataWriter.cxx

IO/XML/Testing/Cxx/TestXMLMappedUnstructuredGridIO.cxx

Imaging/Morphological/vtkImageConnectivityFilter.cxx

Rendering/Context2D/Testing/Cxx/TestContext2D.cxx

.../Core/Testing/Cxx/TestCompositePolyDataMapper2Picking.cxx