Bugfix: Remove implicit path `/restic` for the s3 backend

The s3 backend used the subdir `restic` within a bucket if no explicit path
after the bucket name was specified. Since this version, restic does not use
this default path any more. If you created a repo on s3 in a bucket without
specifying a path within the bucket, you need to add `/restic` at the end of
the repository specification to access your repo: `s3:s3.amazonaws.com/bucket/restic`

https://github.com/restic/restic/pull/1437
https://github.com/restic/restic/issues/1292
