postrm is in update-notifier 0.154.1.
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 8 9 | #!/bin/sh
set -e
STAMP=/var/lib/update-notifier/dpkg-run-stamp
if [ "$1" = purge -a -f $STAMP ]; then
rm $STAMP
fi
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