To remove John the Ripper as well though, the immediate fix would be to delete (also as root):
/var/lib/dpkg/info/john.prerm

And then proceed with uninstalling it as usual.

(Don't get in the habit of doing this, because the various dpkg scripts that come with a package could be doing something important, but in this case I looked through the scripts and it looks like prerm just stops John the Ripper cron jobs that you might have running, so it should be safe in your case.)

(Though the long-term fix would be for me or someone else to patch the cron-job handling script to not break all over the place.)