Removing references from parents

```javascript
var Foo = Backbone.AssociatedModel.extend({});

var Bar = Backbone.AssociatedModel.extend({
       relations: [{
       type: Backbone.One,
            key: 'rel',
            relatedModel: Foo
       }]
});
```
Create some instances:
```javascript
var foo1 = new Foo;
var foo2 = new Foo;

var bar = new Bar({rel: foo1});
```
Now foo1.parents contains one element (bar).
Next, change rel attribute:
```javascript
bar.set({rel: foo2})
```
Conceptually, bar must retire from foo1.parents, but it is'nt.
```
foo1.parents.length => 1
foo2.parents.length => 1
```