Revert "configure: generate an error in extra checks when return type is bogus"

This reverts commit 4cd7d490.

This warning can occur in callbacks that never return. Some functions cannot have a return statement because that would cause dead code warnings, while they also cannot return void because the callback prototype is externally mandated. Ideally the compiler would notice that the function does not return, and sometimes it does, but evidently we cannot rely on that, so we have to revert this warning back to a warning, not an error.

Fixes #28650 (closed).