Bugfix: Delete files on Backblaze B2 more reliably

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

The implementation has now been changed to delete all versions of files, which
doubles the amount of Class B transactions necessary to delete files, but
assures that no file versions are left behind.

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