Clear out import queues for obsolete Ubuntu releases

Once an Ubuntu release is obsolete, there's no need whatsoever to keep its import queue entries around. Delete them with:

DELETE FROM TranslationImportQueueEntry Entry
USING Distribution Ubuntu, DistroSeries
WHERE
    Ubuntu.name = 'ubuntu' AND
    DistroSeries.distribution = Ubuntu.id AND
    DistroSeries.id = Entry.distroseries AND
    -- Status 6 is OBSOLETE.
    DistroSeries.releasestatus = %(obsolete)s;