Remove organisation_id from StatisticAnnouncement

**WARNING: Don not merge until https://github.com/alphagov/whitehall/pull/1751 is successfully deployed to production:**

the change above introduces a join table to store M-M relations between statistics announcements and organisations. hence this column is not required.
There is little point in defining the `down` method as the `up` is destructive. Better to not define it so that rails correctly identifies it as an `ActiveRecord::IrreversibleMigration`.