prerm is in rxvt-unicode-lite 9.19-1.
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 | #!/bin/sh
set -e
if [ "$1" = remove ]; then
    update-alternatives --remove rxvt /usr/bin/urxvt
    update-alternatives --remove x-terminal-emulator /usr/bin/urxvt
fi
 |