postrm is in groonga-munin-plugins 6.0.1-1ubuntu1.
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" = "purge" ]; then
rm -f /etc/munin/plugins/groonga_* > /dev/null 2>&1
fi
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