`this.model.collection.remove(this.model)` removes the chapter from the textbook's chapterset. `this.remove()` removes the view's HTML from the page. We could listen for `remove` events on the chapterset, and when one occurs, destroy all the existing chapter views and re-render all the chapters in the chapterset post-removal -- but I figured that it would be simpler and faster to just remove the one view's HTML when the model backing that view was removed from its collection.