Bugfix: Reliably delete files on Backblaze B2

Restic used to only delete the latest version of files stored in B2. In most
cases this works well as there is only a single version. However, due to
retries while uploading it is possible for multiple file versions to be stored
at B2. This can lead to various problems if files still exist that should have
been deleted.

We have changed the implementation to delete all of them. This doubles the
number of Class B transactions necessary to delete files.

https://github.com/restic/restic/issues/3161
https://github.com/restic/restic/pull/3885
