Destroy backbone model on comm:close

Trigger the `destroy` event in backbone.js before deleting the model. 

Some other objects may still hold references to the model because they have registered to its events. In the implementation of backbone models, one can register an `on('destroy')` handler. 
I'm a little bit confused, why `this.comm.model`?  Isn't that just `this` in the context that this method is called in? In otherwords, `this.trigger('destroy', this)` doesn't work?