Drop ftp.fau.de, currently broken

Drop ftp.fau.de, currently broken

Quoting from http://ftp.fau.de/:

| Service impairment: The machine has a faulty RAID controller. Until the
| shipment of the replacement part arrives, bad performance and crashes
| are to be expected. Mirrors will NOT be updated automatically for now,
| and mostly remain at the version they had at around 2020-06-03 14:30
| UTC. Sorry for the inconvenience. We will post updates on our blog:
| https://blogs.fau.de/ftp/2020/06/04/unplanned-outage-on-june-03/

According to the blog the service should be restored by 2020-06-08
evening, then we can revert this change.