postrm is in adduser 3.113ubuntu2.
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  | #! /bin/sh
set -e
if [ "$1" = "purge" ]; then
    rm -f /etc/adduser.conf /etc/adduser.conf.dpkg-save
    if [ -e /usr/share/debconf/confmodule ]; then
        . /usr/share/debconf/confmodule
        db_purge
    fi
fi
 |