Remove a reference to managed_by_transition from an import rake task

- A transtion-postgres Errbit error (https://errbit.preview.alphagov.co.uk/apps/53e8c4e40da115dfc200684d/problems/5425716e0da11550d7001181) made me do a full search everywhere. Clearly nothing has used
  this rake task since https://github.com/alphagov/transition/pull/413 otherwise
  Errbit would have caught the exception?