postrm is in squirrelmail-compatibility 2.0.16-1.
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  | #!/bin/sh
set -e
# Source debconf library.
case "$1" in
	remove)
		if [ `which squirrelmail-compatibility` ]
		then
			squirrelmail-configure --remove-plugin compatibility
		fi
	;;
	*)
	;;
esac
exit 0
 |