cleanup method don't unbind _proxyCallback from a relationValue

```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});

bar2.cleanup()
// foo.parents.length => 1
// foo._events.all.length => 2 (_proxyCallback don't retired)
```