Deprecate FAI's make-fai-nfsroot.conf

Since FAI v4.0 make-fai-nfsroot.conf is no longer relevant, even in
Debian oldoldstable AKA buster we have FAI v5.8.4, so there's no point
in further supporting this.

Sadly we still need the temporary /etc/grml/fai/nfsroot.conf for FAI's
dirinstall usage:

| fai -C /etc/grml/fai -s file:////etc/grml/fai/config -cDEBIAN_BOOKWORM,GRMLBASE,GRML_SMALL,AMD64 - u grml dirinstall ....

FAI relies on this file, otherwise failing with:

| /etc/grml/fai/nfsroot.conf not found.
| You may want to install the package fai-server