postrm is in hdparm 9.37-0ubuntu3.
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  | #!/bin/sh
set -e
case "$1" in
  purge)
  [ ! -L /etc/udev/rules.d/z60_hdparm.rules ] || rm /etc/udev/rules.d/z60_hdparm.rules
  ;;
esac
 |