Commit d4ee3023 and its parent have the history behind this code.
Since d4ee3023^, all new PreregistrationUser objects, except those for
realm creation, have a non-None `realm`. Since d4ee3023, any legacy
PreregistrationUsers, with a `realm` of None despite not being for
realm creation, are treated as expired. Now, we ignore them
completely, and remove any that exist from the database.
The user-visible effect is to change the error message for
registration (or invitation) links created before d4ee3023^ to be
"link does not exist", rather than "link expired".
This change will at most affect users upgrading straight from 1.7 or
earlier to 1.8 (rather than from 1.7.1), but I think that's not much
of a concern (such installations are probably long-running
installations, without many live registration or invitation links).
[greg: tweaked commit message]
dbe3576706