well, replacing "pain receptors" with a complete rework of the mammalian brain and nervous system, since just removing pain receptors is a very limited kind of alleviation of suffering. After all, I could remove your literal pain receptors and lock you in a 6x6 cell for your whole life, and you'd still be suffering.)

As an aside, what you're describing here would be (to my mind) ethically indistinguishable from vat-grown meat.