prerm is in postgresql-filedump 9.3.0-3.
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  | #!/bin/sh
set -e
case "$1" in
    remove|deconfigure)
	    update-alternatives --quiet --remove pg_filedump /usr/lib/postgresql/9.4/bin/pg_filedump
	    ;;
    *)
esac
 |