fix: revert `Nat.sub` embedding into `Int`

This PR reverts the way `grind cutsat` embeds `Nat.sub` into `Int`. It fixes a regression reported by David Renshaw on Zulip.

https://leanprover.zulipchat.com/#narrow/channel/113488-general/topic/v4.2E23.2E0/near/534646557
4 files changed