Remove unused Row class in SelectManager

The `Row` class is no longer used since d956772b3c.

activerecord/lib/arel/select_manager.rb

@@ -237,18 +237,6 @@ module Arel # :nodoc: all
       @ctx.source
     end
 
-    class Row < Struct.new(:data) # :nodoc:
-      def id
-        data["id"]
-      end
-
-      def method_missing(name, *args)
-        name = name.to_s
-        return data[name] if data.key?(name)
-        super
-      end
-    end
-
     private
       def collapse(exprs)
         exprs = exprs.compact