postrm is in xen-hypervisor-4.6-amd64 4.6.0-1ubuntu4.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | #!/bin/bash
set -e
case "$1" in
    remove)
        if command -v update-grub > /dev/null && [ -d /boot/grub ]; then
            update-grub || :
        fi
    ;;
    purge|upgrade|failed-upgrade|abort-install|abort-upgrade|disappear)
    ;;
    *)
        echo "postrm called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac
exit 0
 |