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.
Remove the redirection_date line from all existing site YAML

I paired with @rjc123 on this to do it in a script rather than me
spending quite a while removing them manually:

`while read line; do grep -v redirection_date $line >
site_list_redirection_date.txt; mv site_list_redirection_date.txt $line;
done < site_list.txt`
Merge pull request #918 from alphagov/remove_transition_date_from_config

Remove references to redirection_date