postrm is in acpid 1:2.0.21-1ubuntu2.
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 | #!/bin/sh
set -e
if [ "$1" = purge ]; then
	rm -f /var/backups/acpid-cruft.tar.gz
fi
 |