"No longer spawn" means that if you go down to the Loc Muinne sewers, to the place where bullvores appeared in the earlier versions, there are no longer any bullvores there. They just do not appear. And if you were expecting them for this quest, you will be disappointed.
On PC, you can use the mod and buy the bullvore brain from Lockhart. On Xbox 360, I don't have a useful answer for you.