Move each of the sites in the directgov_legacy block into its own file

- I think this is all of them - one was a www. alias of another, so
  added to the same file, if the counts between these files and
  https://github.com/alphagov/redirector/blob/master/data/transition-sites/directgov_legacy.yml
  don't match up.
- Remove `directgov_legacy.yml`.