Only purge /etc/default/grub if it's owned in ucf by same package.
On d-boot it was reported that we remove /etc/default/grub on purge. Even when switching from e.g. grub-efi-amd64 back to grub-pc
Keep it avaible if the file is in ucf registered to a different package then the one we purge
Long Goal would be maybe to move /etc/default/grub completely to grub2-common