_proxyCallback can't be off

```javascript
var Foo = Backbone.AssociatedModel.extend({});

        var Bar = Backbone.AssociatedModel.extend({
            relations: [
                {
                    type: Backbone.One,
                    key: 'rel',
                    relatedModel: Foo
                }
            ],
        });

        var foo = new Foo;

        var bar1 = new Bar({rel: foo});
        var bar2 = new Bar({rel: foo})

        // foo.parents.length => 2
        // foo._events.all.length => 2 (proxyCallback's for calling bubbleEvent)
        bar1.destroy();

        // foo.parents.length => 1
        // foo._events.all.length => 2 // bug
```