Remove download option for on-the-fly compressed backups

Starting with version 6.1, it will not be possible any longer to download compressed backup files (they can be identified by the suffix ".tar.gz") using the cluster backup interfaces to retrieve complete backups as one compressed file.

Background: These files are created on-the-fly and downloading them proved to be very slow, especially in larger installations. For guidance on how to download full backup sets from clusters, please refer to our example script here: https://github.com/EXASOL/exaoperation-xmlrpc/tree/master/tools-and-examples/download_backupset