Drop depends on fai-server

Drop depends on fai-server

We don't seem to actually need the fai-server package, which ships the
following binaries:

* /usr/bin/fai-mirror/usr/bin/fai-mk-configspace
* /usr/bin/fai-monitor-gui
* /usr/sbin/dhcp-edit
* /usr/sbin/fai-cd
* /usr/sbin/fai-chboot
* /usr/sbin/fai-diskimage
* /usr/sbin/fai-make-nfsroot
* /usr/sbin/fai-mk-network
* /usr/sbin/fai-monitor
* /usr/sbin/fai-new-mac
* /usr/sbin/fai-setup

Furthermore, fai-server ships the following configuration file:

* /etc/fai/NFSROOT
* /etc/fai/apt
* /etc/fai/apt/sources.list
* /etc/fai/apt/trusted.gpg.d
* /etc/fai/apt/trusted.gpg.d/fai-project.gpg
* /etc/fai/grub.cfg
* /etc/fai/grub.cfg.autodiscover
* /etc/fai/nfsroot.conf

We might still need some variant of /etc/fai/nfsroot.conf
via `${GRML_FAI_CONFIG}/nfsroot.conf` +
`${GRML_FAI_CONFIG}/make-fai-nfsroot.conf`, though if we could finally get rid
of also those, that would be certainly be nice.

Thanks: András Korn