Remove concurrent-ruby from Action Pack gemspec

It is already on Active Support

Author: rafaelfranca

actionpack/actionpack.gemspec

@@ -26,7 +26,6 @@ Gem::Specification.new do |s|
   s.add_dependency 'rails-html-sanitizer', '~> 1.0', '>= 1.0.2'
   s.add_dependency 'rails-dom-testing', '~> 1.0', '>= 1.0.5'
   s.add_dependency 'actionview', version
-  s.add_dependency 'concurrent-ruby', '~> 0.9.0'
 
   s.add_development_dependency 'activemodel', version
 end