rm `ActiveRecord::Type::Decorator`

It only existed to make sure the subclasses of `Delegator` were YAML
serializable. As of Ruby 2.2, these are YAML dumpable by default, as it
includes
https://github.com/tenderlove/psych/commit/2a4d9568f7d5d19c00231cf48eb855cc45ec3394

activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

@@ -2,8 +2,6 @@ module ActiveRecord
   module AttributeMethods
     module TimeZoneConversion
       class TimeZoneConverter < DelegateClass(Type::Value) # :nodoc:
-        include Type::Decorator
-
         def type_cast_from_database(value)
           convert_time_to_time_zone(super)
         end

activerecord/lib/active_record/type.rb

 require 'active_record/type/helpers'
-require 'active_record/type/decorator'
 require 'active_record/type/mutable'
 require 'active_record/type/numeric'
 require 'active_record/type/time_value'

activerecord/lib/active_record/type/decorator.rb

-module ActiveRecord
-  module Type
-    module Decorator # :nodoc:
-      def init_with(coder)
-        @subtype = coder['subtype']
-        __setobj__(@subtype)
-      end
-
-      def encode_with(coder)
-        coder['subtype'] = __getobj__
-      end
-    end
-  end
-end

activerecord/lib/active_record/type/serialized.rb

@@ -2,7 +2,6 @@ module ActiveRecord
   module Type
     class Serialized < DelegateClass(Type::Value) # :nodoc:
       include Mutable
-      include Decorator
 
       attr_reader :subtype, :coder
 
@@ -36,16 +35,6 @@ def accessor
         ActiveRecord::Store::IndifferentHashAccessor
       end
 
-      def init_with(coder)
-        @coder = coder['coder']
-        super
-      end
-
-      def encode_with(coder)
-        coder['coder'] = @coder
-        super
-      end
-
       private
 
       def default_value?(value)