This should do the trick:

public class Script1 : RAScriptClass91
    {
        protected override void Execute()
        {
            using (DocumentInfo di = new DocumentInfo(BoundEntryId,RASession))
            {
                List<AnnotationBase> anns = (List<AnnotationBase>)di.GetAnnotations();
                foreach(AnnotationBase ann in anns)
                {
                    if (ann.AnnotationType == AnnotationType.Note) Annotation.Delete(ann.EntryId,ann.PageNumber,ann.ItemId,RASession);
                }
                di.Save();
            }
        }
    }