Fix registrations on AAIB reports

[Trello ticket](https://trello.com/c/idhIrT04/367-investigate-registration-and-or-registration-string-on-aaib-reports-2)

The import script added both registration (an array) and registration
string (as string) to extra fields. Desired behaviour was to just use
string.
Removed registration array, and renamed registration string to
registration.