Remove the advice to run the import:hits_mappings_relations rake task

- In writing some instructions on how to generate mappings from hits for
  sites in preview and production, I wondered about the
  `import:hits_mappings_relations` rake task and its purpose. Following
  feedback from Jenny, it seems like the purpose of this rake task is
  fulfilled by
  https://github.com/alphagov/transition/blob/master/app/models/mapping.rb#L42.
  This means that the advice at the end of `import:mappings_from_host_paths`
  to run it can be removed.
- The rake task itself is being kept and its description updated to
  "Dev only" because it's still useful in dev.