TT#61152 Drop support of debootstrap

Since mr7.3 we use mmdebstrap as it's faster so it's time to
drop debootstrap support.

Change-Id: I0cd5b03ace2b752decd71c4a06d8109105fb5c1e

templates/scripts/includes/deployment.sh

@@ -1390,38 +1390,18 @@ deb ${MIRROR} ${DEBIAN_RELEASE}-updates main contrib non-free
 deb ${DBG_MIRROR} ${DEBIAN_RELEASE}-debug main contrib non-free
 EOF
 
-case "$DEBIAN_RELEASE" in
-  stretch|buster)
-    if ! [ -r "/usr/share/debootstrap/scripts/${DEBIAN_RELEASE}" ] ; then
-      echo "Enabling ${DEBIAN_RELEASE} support for debootstrap via symlink to sid"
-      ln -s /usr/share/debootstrap/scripts/sid "/usr/share/debootstrap/scripts/${DEBIAN_RELEASE}"
-    fi
-    ;;
-esac
-
-# defaults
-DEBOPT_OPTIONS=("--keyring=${KEYRING} --no-merged-usr")
-if checkBootParam nommdebstrap ; then
-  echo "Boot option nommdebstrap found, disabling usage of mmdebstrap for installing Debian"
-else
-  # mmdebstrap is available only since buster, so ensure we're running on
-  # a buster based Grml ISO
-  case $(cat /etc/debian_version) in
-    buster*|10*)
-      echo "Using mmdebstrap for bootstrapping Debian"
-      ADDITIONAL_PACKAGES+=(mmdebstrap)
-      ensure_packages_installed
-      export DEBOOTSTRAP=mmdebstrap  # for usage with grml-debootstrap
-      # it's a no-op in mmdebstrap v0.4.1, but force its usage to not be surprised
-      # if that default should ever change
-      DEBOPT_OPTIONS=("--no-merged-usr")
-      ;;
-    *)
-      echo "NOTE: not running on top of a Debian/buster based ISO, can't enable mmdebstrap usage"
-      ;;
-  esac
+if [[ ! -x "$(which mmdebstrap)" ]]; then
+  die "Can't find mmdebstrap"
 fi
 
+ADDITIONAL_PACKAGES+=(mmdebstrap)
+ensure_packages_installed
+export DEBOOTSTRAP=mmdebstrap  # for usage with grml-debootstrap
+
+# it's a no-op in mmdebstrap v0.4.1, but force its usage to not be surprised
+# if that default should ever change
+DEBOPT_OPTIONS=("--no-merged-usr")
+
 # install only "Essential:yes" packages plus apt (explicitly included in minbase variant),
 # systemd + network related packages
 DEBOPT_OPTIONS+=("--variant=minbase --include=systemd,systemd-sysv,init,isc-dhcp-client,ifupdown")