prerm is in openjdk-9-jdk 9~b114-0ubuntu1.
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  | #!/bin/sh -e
set -e
jdk_tools='appletviewer jconsole'
basedir=/usr/lib/jvm/java-9-openjdk-amd64
if [ "$1" = "remove" ] || [ "$1" = "deconfigure" ]; then
    for i in $jdk_tools; do
	update-alternatives --remove $i $basedir/bin/$i
    done
fi
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