postrm is in xscreensaver-gl 5.15-2ubuntu1.
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 | #!/bin/sh
set -e
case "$1" in 
  remove|purge)
	if [ -L /etc/X11/app-defaults/XScreenSaver ]; then
	  rm /etc/X11/app-defaults/XScreenSaver
	  if [ -f /etc/X11/app-defaults/XScreenSaver-nogl ]; then
	    ln -sf XScreenSaver-nogl /etc/X11/app-defaults/XScreenSaver
	  fi
	fi
	if [ -L /usr/X11R6/lib/X11/app-defaults/XScreenSaver ]; then
		rm /usr/X11R6/lib/X11/app-defaults/XScreenSaver
	fi
	if [ -L /usr/doc/xscreensaver-gl ]; then
		rm -f /usr/doc/xscreensaver-gl
	fi
	;;
esac
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