Werk #15042: mkbackup: Do not list system backups (appliance only)

When running in the Checkmk Appliance, the command line tool for administrating site backups also listed system backups (mkbackup executed as a site user). This is no longer the case, since system backups cannot be used for a site restore. This is consistent with the user interface, which also lists site backups only.

Note that this does not impact the option use system backup targets for site backup jobs.

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