postrm is in nautilus 1:3.26.3-0ubuntu4.
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 | #!/bin/sh -e
if [ "$1" = "purge" ] ; then
    if [ -d /etc/X11/starthere ]; then
        rmdir --ignore-fail-on-non-empty /etc/X11/starthere
    fi
    if [ -d /etc/X11/sysconfig ]; then
        rmdir --ignore-fail-on-non-empty /etc/X11/sysconfig
    fi
    if [ -d /etc/X11/serverconfig ]; then
        rmdir --ignore-fail-on-non-empty /etc/X11/serverconfig
    fi
fi
 |