How do you delete a 3DS (that you no longer own) from your Nintendo Network ID?

I recently  traded  in my 3DS XL to GameStop when I preordered  the NEW 3DS XL.

I've been trying to link my NEW 3DS XL to my Nintendo Network ID, but its not letting me. It is saying that my old 3ds is still connected to my Nintendo Network ID.

I can't   transfer my ID, because I don't have my old 3DS, so how do I delete my old 3ds xl from my Nintendo Network ID....so I can add my New 3DS XL  to it?