No longer bootstrap with --no-merged-usr

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