Remove attribute_name_for

This hasn't been used since 2012, and was only ever used by other helper
methods (probably why it was private, though that doesn't really
restrict usage).

backend/app/helpers/spree/admin/base_helper.rb

@@ -130,12 +130,6 @@ def admin_layout(layout = nil)
         @admin_layout = layout if layout
         @admin_layout
       end
-
-      private
-
-      def attribute_name_for(field_name)
-        field_name.tr(' ', '_').downcase
-      end
     end
   end
 end