Remove references to redirection_date

- Remove the redirection_date line from the generated file from the
  new_site rake task.
- This is part of reconfiguring transition to be the canonical source of
  redirection (transition) dates, in an attempt to keep them up to date
  without developer intervention.

(This is related to https://github.com/alphagov/transition/pull/396, which should probably be merged first?)