Let me add to that. I might be wrong to insist that you can't delete them, and I'll consider changing that. I would just like a bit more to go on. I understand that you don't want the nodes and you are assigning a completely different set of global shaders to lots of different objects. That's perfectly fine and it's important that you can do that. It's by design you can do that. But as for completely deleting the Parts Shader, there are reasons why I think it should stick around, just in case, because it's the only way to reconnect individual parts associated with specific objects, in general. (It's awkward that it's handled using "Parts" nodes in the first place, but don't have a better way of presenting dynamic arrays of connections yet.) Leaving the unused nodes around may be completely useless to you and other people in many cases, I understand that. The need to delete them, is it out of concern for efficiency, file size, general nagging feeling that they are sitting there for no reason, aesthetics, readability (if you're editing externally). Any of the above, or something else? Do the Parts Shader or the Part nodes prevent the old assignment method from working? If so, I didn't realise that, and I would have liked someone to tell me.
I might want to make the change if I know a bit more about your reasons, but I want to weight it against other considerations.
Matt