Lower tier registrations get an expires_on date when seeded

[andrewhick]

Currently, when a lower tier registration gets seeded, it also gets given an expiry date, which it should not have in live.

This results in some strange test behaviour - such as renewal letters being generated for lower tier registrations.

Although this is not causing current tests to fail, the expires_on date should only be set for upper tier registrations, to make test data match production.