Remove now-redundant migration.

It was used to migrate between 2 data representations.  It was also the
only thing using the :location param in Place.create_from_hash
Remove redundant import task

This presumably predates the admin upload interface.  It also allows us
to clean up some stuff from the Rakefile
Remove setting of GOVUK_* env vars in Rakefile

This is no longer necessary after plek 1.1.0
Merge pull request #56 from alphagov/cleanup_rake_tasks

Cleanup some redundant rake tasks