postrm is in initramfs-tools-core 0.122ubuntu8.14.
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 | #!/bin/sh
set -e
if [ "x${1}" = "xpurge" ]; then
rm -f /etc/initramfs-tools/conf.d/resume
rm -f /etc/initramfs-tools/modules
fi
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