comments_template() is unused

IDatasetForm has a `comments_template()` method (with a corresponding implementation in `DefaultPackageForm` but it never gets called. There's a `_comments_template()` in `PackageController` that looks up the IDatasetForm's comments_template() method, but _comments_template() is never called.

I'm not sure what this is for. Remove it?

(If so it should also be removed from ExampleIDatasetForm and its tests)