remove z-push/kopano (source code unavailable)

- the source code links returns an error 403
- https://kb.kopano.io/display/ZP/How+to+get+the+latest+Z-Push+code+from+GIT points to https://stash.z-hub.io/scm/zp/z-push.git which times out
- https://z-push.org/download/ indicates there are no releases since 2021
- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/issues/2266

README.md

@@ -885,7 +885,6 @@ _Related: [Groupware](#groupware)_
 - [Seafile](https://www.seafile.com/en/home/) - File hosting and sharing solution primary for teams and organizations. ([Source Code](https://github.com/haiwen/seafile)) `GPL-2.0/GPL-3.0/AGPL-3.0/Apache-2.0` `C`
 - [Syncthing](https://syncthing.net/) - Syncthing is an open source peer-to-peer file synchronisation tool. ([Source Code](https://github.com/syncthing/syncthing)) `MPL-2.0` `Go`
 - [Unison](https://www.cis.upenn.edu/~bcpierce/unison/) - Unison is a file-synchronization tool for OSX, Unix, and Windows. ([Source Code](https://github.com/bcpierce00/unison)) `GPL-3.0` `OCaml`
-- [Z-Push](https://z-push.org/) - Implementation of Microsoft’s ActiveSync protocol. ([Source Code](https://stash.kopano.io/projects/ZHUB/repos/z-push/browse)) `AGPL-3.0` `PHP`
 
 
 ### File Transfer - Distributed Filesystems