backups: remove zbackup (unmaintained since 2016)

- https://github.com/zbackup/zbackup/commits/master
- ref. https://github.com/awesome-foss/awesome-sysadmin/issues/405

README.md

@@ -115,7 +115,6 @@
 - [Shield](https://github.com/starkandwayne/shield) - A pluggable architecture for backup and restore of database systems.
 - [Snebu](https://www.snebu.com/) – Snapshot backup with global multi-client deduplication and transparent compression. ([Source Code](https://github.com/derekp7/snebu/))
 - [UrBackup](https://www.urbackup.org/) - Another client-server backup system. ([Source Code](https://github.com/uroni/urbackup_backend))
-- [ZBackup](http://zbackup.org/) - A versatile deduplicating backup tool. ([Source Code](https://github.com/zbackup/zbackup))
 
 
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