Remove unnecessary dup from FormTagHelper#field_id

This was needed when `sanitized_method_name` was a memoised method, but
since 59ca21c it's a local variable.

Author: eugeneius

actionview/lib/action_view/helpers/form_tag_helper.rb

@@ -104,7 +104,7 @@ def field_id(object_name, method_name, *suffixes, index: nil)
 
         # a little duplication to construct fewer strings
         if sanitized_object_name.empty?
-          sanitized_method_name.dup
+          sanitized_method_name
         elsif suffixes.any?
           [sanitized_object_name, index, sanitized_method_name, *suffixes].compact.join("_")
         elsif index