rubocop-rspec-unused-let

https://github.com/tk0miya/rubocop-rspec-unused-let

rubocop-rspec-unused-let adds the RSpec/UnusedLet cop, which flags `let` (and optionally `let!`) definitions, `subject` definitions and helper methods (`def`) that are never referenced within their scope. It resolves `shared_examples` precisely when it can see the shared block, and stays conservative otherwise to avoid false positives.