ENH: Clean up vtkMRMLLinearTransformsDisplayableManager implementation

Refactor and combine both 2D and 3D displayable managers for linear transform nodes into a single class.

Since both 2D and 3D displayable managers are now generating the exact same interaction widget, they ended up sharing all of the same code.
To avoid duplication and to reduce maintenance, this commit combines the two displayable managers into a single class.

Re #7570

Modules/Loadable/Transforms/MRMLDM/CMakeLists.txt

@@ -15,8 +15,7 @@ set(${KIT}_INCLUDE_DIRECTORIES
 set(displayable_manager_SRCS
   vtkMRML${MODULE_NAME}DisplayableManager2D.cxx
   vtkMRML${MODULE_NAME}DisplayableManager3D.cxx
-  vtkMRMLLinear${MODULE_NAME}DisplayableManager3D.cxx
-  vtkMRMLLinear${MODULE_NAME}DisplayableManager2D.cxx
+  vtkMRMLLinear${MODULE_NAME}DisplayableManager.cxx
   vtkMRMLTransformHandleWidget.cxx
   vtkMRMLTransformHandleWidgetRepresentation.cxx
   )