Comment 2 for bug 373351

The librarian URL for the deleted attachment also appears in the
activity log, so we must ignore all attachment changes in private
bugs. Unfortunately, this also reduces the usefulness of private bugs.

Perhaps a different solution here is to make sure the librarian either
deletes or denies access for deleted files? A new "purge" mechanism
could be added to the librarian (if one doesn't already exist) for
this use case.