postrm is in gnokii-cli 0.6.31+dfsg-2ubuntu1.
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  | #!/bin/sh
set -e
# Old versions added a gnokii user too, not needed anymore so it can 
# be removed on upgrade too.
if getent passwd gnokii; then
    if [ $1 = "purge" ]; then
        userdel --remove gnokii
    else
        userdel gnokii
    fi
fi
case "$1" in
  remove|purge)
    if getent group gnokii; then
      dpkg-statoverride --quiet --remove /usr/sbin/mgnokiidev || true
      groupdel gnokii
    fi
esac
 |