Remove Adapter autoloads in favor of require

Adapters must be eager loaded to ensure they are defined
before they are used as namespacing.

cf6a074a1c (diff-41f2b3509d33e1c65bb70ee0ec7a2eea)

lib/active_model/serializer.rb

@@ -9,8 +9,6 @@ require 'active_model/serializer/utils'
 
 module ActiveModel
   class Serializer
-    extend ActiveSupport::Autoload
-
     include Configuration
     include Associations