postrm is in software-center 13.10-0ubuntu4.
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 14 15 16 | #!/bin/sh
set -e
# remove database files from the old location 
# (it was named software-store for some weeks)
if [ -d /var/cache/software-store/xapian/ ]; then
    rm -f /var/cache/software-store/xapian/*
    rmdir /var/cache/software-store/xapian/ || true
fi
if [ "$1" = "purge" ]; then
    # remove the cached application data
    rm -f /var/cache/software-center/xapian/*
    rmdir /var/cache/software-center/xapian/
fi
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