This will remove the `builderView` property from the `FormView` object, but that doesn't guarantee it'll get cleaned up properly. Since we've bound something to an event on the `BuilderView` object, we might retain a reference.

We can use `this.listenTo(this.builderView, 'done', function () {})` and then later `this.stopListening(this.builderView)` to make sure we don't attach to events of an irrelevant object and end up leaking it. Then when we're done with the `BuilderView` instance, we can just set `this.builderView = null`.

Backbone also has a `remove` method on `View`, but that'll remove the whole node from the DOM, which means we'd have to recreate the root element of that node next time we create a `BuilderView`.