TT#60950 Remove stretch labels from deployment.sh (trunk is buster now)

I am not removing "case" code blocks here as they
might become necessary again in bullseye.

Change-Id: Id5f56fe72b0d6b90dc5ae1dd5504f81314ae07ff

templates/scripts/includes/deployment.sh

@@ -1391,7 +1391,7 @@ deb ${DBG_MIRROR} ${DEBIAN_RELEASE}-debug main contrib non-free
 EOF
 
 case "$DEBIAN_RELEASE" in
-  stretch|buster)
+  buster)
     if ! [ -r "/usr/share/debootstrap/scripts/${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}"
@@ -1801,7 +1801,7 @@ EOT
 fi
 
 case "$DEBIAN_RELEASE" in
-  stretch|buster)
+  buster)
     set_custom_grub_boot_options
     ;;
 esac
@@ -2109,7 +2109,7 @@ puppet_install_from_puppet () {
   chroot $TARGET apt-get -y install resolvconf libnss-myhostname
 
   case "$DEBIAN_RELEASE" in
-    stretch|buster)
+    buster)
       if [ ! -x "${TARGET}/usr/bin/dirmngr" ] ; then
         echo  "Installing dirmngr on Debian ${DEBIAN_RELEASE}, otherwise 'apt-key adv --recv-keys' is failing to fetch GPG key"
         chroot $TARGET apt-get -y install dirmngr
@@ -2140,7 +2140,7 @@ EOF
 
   # Fix Facter error while running in chroot, facter fails if /etc/mtab is absent:
   case "$DEBIAN_RELEASE" in
-    stretch|buster)
+    buster)
       chroot ${TARGET} ln -s /proc/self/mounts /etc/mtab || true
       ;;
   esac