postrm is in piuparts-slave 0.64ubuntu1.
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 24 | #!/bin/sh
set -e
user=piupartss
if [ "$1" = "purge" ] ; then
    rm -rf /var/lib/piuparts/$user
    if [ -d /var/lib/piuparts/slave ] ; then
        rm -rf /var/lib/piuparts/slave
    fi
    rmdir /var/lib/piuparts 2>/dev/null || true
    rmdir /etc/piuparts 2>/dev/null || true
    rm -rf /var/cache/piuparts/basetgz/*.log
    rm -rf /var/cache/piuparts/basetgz/*.tar.gz
    # rm -rf is safe if it's not mounted anymore
    mount | grep -q /var/cache/piuparts/tmp || rm -rf --one-file-system /var/cache/piuparts/tmp
    rmdir /var/cache/piuparts 2>/dev/null || true
fi
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