Remove managed_by_transition from the hosts API

- In doing this, the hosts API no longer knows about
  managed_by_transition, and no longer refers to a site of a host being
  managed by transition. We're removing two hosts features that
  reference errors in the JSON, as there will be no errors now because
  it is just a list of hosts. Jenny and I discussed making the hosts API
  error when there is no data, but that would be inconsistent with the
  rest of the app's behaviour.