Remove the deprecated add_child method

[DoctorJohn]

This was removed because it was essentially implementing set-like features on top of a list (i.e., using a set would have been easier). That being said, neither this approach nor using a set would currently work because of the recursive nature of the Feature model. To make things work, we would have to add a custom __eq__ method to the Feature model. But it looks like we don't need this behavior at all, so let's remove it.