Revert "Double check that passes correctly set their Modified status"

This check fires during self-host.

> The approach is simple: if a pass reports that it's not modifying a
> Function/Module, compute a loose hash of that Function/Module and compare it
> with the original one. If we report no change but there's a hash change, then we
> have an error.
>
> This approach misses a lot of change but it's not super intrusive and can
> detect most of the simple mistakes.
>
> Differential Revision: https://reviews.llvm.org/D80916

This reverts commit 3667d87a.