fix: unset initrdless_boot_fallback_triggered instead of setting it to 0

This fixes GCE shielded VM instances integrity monitoring failures on
focal and later. Our images are built with an empty /boot/grub/grubenv
file, however after the first boot `initrdless_boot_fallback_triggered`
is set to 0. This change in `grubenv` results in integrity monitoring
`lateBootReportEvent` error.

It seems that the only thing that's checking for this `grubenv` variable
is `grub-common.service`, and it is looking specifically for a `1`
value:

  if grub-editenv /boot/grub/grubenv list | grep -q
  initrdless_boot_fallback_triggered=1; then echo "grub:
  GRUB_FORCE_PARTUUID set, initrdless boot paniced, fallback triggered.";
  fi

Unsetting this variable instead of setting it to 0 would prevent issues
with integrity monitoring.

live-build/functions

@@ -1137,7 +1137,7 @@ if [ -n "${have_grubenv}" ]; then
   if [ -n "${initrdfail}" ]; then
     set initrdless_boot_fallback_triggered="${initrdfail}"
   else
-    set initrdless_boot_fallback_triggered=0
+    unset initrdless_boot_fallback_triggered
   fi
   save_env initrdless_boot_fallback_triggered
 fi