Drop development dependency on Rack gem

This PR drops development dependency on Rack gem.

This dependency was introduced at rubocop#5597 for `Rails/HttpStatus` cop.

Rails cops has been removed in rubocop#7095 from RuboCop core and the
dependency on Rack gem is maintained by RuboCop Rails repo.
rubocop/rubocop-rails#68

rubocop.gemspec

@@ -41,5 +41,4 @@ Gem::Specification.new do |s|
  s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 1.7')
 
  s.add_development_dependency('bundler', '>= 1.15.0', '< 3.0')
- s.add_development_dependency('rack', '>= 2.0')
 end