remove unused require ‘set’

actionpack/lib/action_controller/caching.rb

 require 'fileutils'
 require 'uri'
-require 'set'
 
 module ActionController
   # \Caching is a cheap way of speeding up slow applications by keeping the result of

activerecord/lib/active_record/base.rb

 require 'yaml'
-require 'set'
 require 'active_support/benchmarkable'
 require 'active_support/dependencies'
 require 'active_support/descendants_tracker'

railties/test/application/initializers/frameworks_test.rb

 require "isolation/abstract_unit"
-require 'set'
 
 module ApplicationTests
   class FrameworksTest < ActiveSupport::TestCase