No longer bootstrap with --no-merged-usr

The transition to usrmerge was done in Debian, see
https://lists.debian.org/debian-devel-announce/2022/09/msg00001.html
Debian/bookworm AKA v12 will only support the merged-/usr layout.

Systemd is also dropping support for unmerged-usr systems (see
https://lists.freedesktop.org/archives/systemd-devel/2022-September/048352.html).

Also debootstrap complains about this nowadays:

| W: Upgrading non-merged-/usr environments post-bookworm is unsupported. Only do this for CI/QA infrastructure that will be re-bootstrapped rather than upgraded.

So no longer enable --no-merged-usr by default, if anyone should
still need it, it can be set via custom $FAI_DEBOOTSTRAP_OPTS.

Thanks: Csillag Tamas for reporting

grml-live

@@ -712,7 +712,7 @@ if [ -z "$FAI_DEBOOTSTRAP" ] ; then
 fi
 
 if [ -z "$FAI_DEBOOTSTRAP_OPTS" ] ; then
-  FAI_DEBOOTSTRAP_OPTS="--exclude=info,tasksel,tasksel-data --include=aptitude --arch $ARCH --no-merged-usr"
+  FAI_DEBOOTSTRAP_OPTS="--exclude=info,tasksel,tasksel-data --include=aptitude --arch $ARCH"
 fi
 
 # create backup of old (not yet automatically generated) config file