Remove organisation_id from StatisticAnnouncement

https://github.com/alphagov/whitehall/pull/1751

the change above introduces a join table to store
M-M relations between statistics announcements and
organisations. hence this column is not required.
Merge pull request #1752 from alphagov/remove-organisation-id-from-statistics-announcements

Remove organisation_id from StatisticAnnouncement