Remove code to generate mappings and tests files

These were only needed for Redirector-as-an-app, and it's now only
a config repository for Transition.
Remove type option to new_site task

We now have only transition-sites for Bouncer, so this option is
redundant.
Merge pull request #1009 from alphagov/simplify_new_site_task

Remove code only needed for Redirector-as-an-app