Remove unused `require "active_support/core_ext/kernel/singleton_class"`

`singleton_class` is in Ruby 1.9.2, and there is no use singleton class
eval in the files.

actionpack/lib/action_dispatch/testing/integration.rb

@@ -2,7 +2,6 @@
 
 require "stringio"
 require "uri"
-require "active_support/core_ext/kernel/singleton_class"
 require "rack/test"
 require "minitest"
 

actionview/lib/action_view/template.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "active_support/core_ext/kernel/singleton_class"
 require "thread"
 require "delegate"
 

activerecord/lib/active_record/scoping/named.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/array"
-require "active_support/core_ext/kernel/singleton_class"
 
 module ActiveRecord
   # = Active Record \Named \Scopes

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "active_support/core_ext/kernel/singleton_class"
 require "active_support/core_ext/module/redefine_method"
 require "active_support/core_ext/array/extract_options"
 

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "erb"
-require "active_support/core_ext/kernel/singleton_class"
 require "active_support/core_ext/module/redefine_method"
 require "active_support/multibyte/unicode"
 

activesupport/lib/active_support/deprecation/instance_delegator.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "active_support/core_ext/kernel/singleton_class"
 require "active_support/core_ext/module/delegation"
 
 module ActiveSupport

railties/lib/rails/generators.rb

@@ -6,7 +6,6 @@ $:.unshift(activesupport_path) if File.directory?(activesupport_path) && !$:.inc
 require "thor/group"
 require "rails/command"
 
-require "active_support/core_ext/kernel/singleton_class"
 require "active_support/core_ext/array/extract_options"
 require "active_support/core_ext/enumerable"
 require "active_support/core_ext/hash/deep_merge"