Cleaning backups in 9.0.13 doesn't seem to remove the .backupinfo files associated with the new backup integrity features of 9.0.

In my case, the backups are encrypted, in case that makes any difference to how the backup info files are generated. So the files are something like app_<app_name>_v3.11.5.tar.gz.enc.backupinfo. I have many of these files remaining after cleaning up some backups. The larger backups are gone as expected thankfully, but the backup info files remain.

I use a s3 backend for my backups (iDrive e3 specifically).

Steps to reproduce

1. Run backups for a period of time.
2. Set a smaller retention window that there are backups and initiate a "Cleanup backups" task from the Backups page.
3. Monitor the s3 storage backend (this may work on other backends too), and watch the files disappear except for the .backupinfo files.