Remove `Enumerator::Chain#initialize_copy`

I think `Enumerator::Chain#initialize_copy` is unnecessary because CRuby
doesn't clone elements.

mrbgems/mruby-enum-chain/mrblib/chain.rb

@@ -20,10 +20,6 @@ class Enumerator
       @enums = args
     end
 
-    def initialize_copy(orig)
-      @enums = orig.__copy_enums
-    end
-
     def each(&block)
       return to_enum unless block_given?
 
@@ -53,11 +49,5 @@ class Enumerator
     def inspect
       "#<#{self.class}: #{@enums.inspect}>"
     end
-
-    def __copy_enums
-      @enums.each_with_object([]) do |e, a|
-        a << e.clone
-      end
-    end
   end
 end