core: drop taints for nobody user/group names

core: drop taints for nobody user/group names

We have a check and warning at compile time. The user cannot do anything about
this at runtime, and all other taints are about checks that happen at runtime
and are specific to that system (and at least potentially correctable).

(The logic in the compilation-time check was updated to treat "nogroup" as OK,
but not the runtime check. But I think it's better to remove the runtime check
for this altogether, so this becomes moot.)