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

- Now we don't use `govuk-lint` to wrap RuboCop, we _could_ switch this
  to run `bundle exec rubocop`, but that's 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.

alphagov/content-store,alphagov/content-store