Currently, unconfirmed registrations remain in the database. Although we
have an expiry for the registrations, we don't actually remove rows from
the database. This can clog the admin interface up with unnecessary
registration spam.

We currently have a patchwork cron script to send notifications on patch
changes, so hook this into a new do_expiry function.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>