postrm is in dctrl-tools 2.23ubuntu1.
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 | #!/bin/sh
set -e
dpkg-maintscript-helper rm_conffile \
/etc/grep-dctrl.rc 2.20.1 dctrl-tools -- "$@"
|