rm unfinished undo/redo engine (#757)

This resolves #757 (closed).

GUI/View/SampleDesigner/SampleEditorController.cpp

@@ -399,11 +399,6 @@ void SampleEditorController::setCurrentIndex(ISelectionContainerForm* widget, in
     emit modified();
 }
 
-QUndoStack* SampleEditorController::undoStack()
-{
-    return &m_undoStack;
-}
-
 MaterialModel* SampleEditorController::materialModel() const
 {
     return &m_sampleItem->materialModel();