Remove `lint` Rake task
Remove `lint` Rake task

- The `govuk-lint` gem is deprecated, the `diff` option hasn't worked
  for a while, and this app doesn't have any SASS to lint.
- While we could switch this to run `bundle exec rubocop`, this `lint`
  task is not very useful as it hides the directories that it operates on,
  and we're trying to move away from wrappers.
- Instead, I think we can leave people to run `bundle exec rubocop` on
  their own - it's less typing than `bundle exec rake lint` and
  `rubocop --help` works for options.

https://trello.com/c/IEUuFOVa/129-remove-the-lint-rake-task-from-apps-that-have-it
as part of the developer tooling group.

alphagov/service-manual-publisher,alphagov/service-manual-publisher,alphagov/service-manual-publisher