prerm is in ldm-lubuntu-theme 2:2.0.47.
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 | #! /bin/sh
set -e
case "$1" in
  remove|deconfigure|failed-upgrade)
    update-alternatives --remove ldm-theme \
        /usr/share/ldm/themes/lubuntu
  ;;
esac
 |