Creating and deleting associations doesn't remove the record from the ManyArray

Here are two tests involving associations.  The first one passes and the second one is failing.  Both of these tests can be added to `packages/ember-data/tests/unit/many_array_test.js`

If an association exists, when it is loaded and then deleted using `deleteRecord()` it is removed from the association.  This is demonstrated in the first test which currently passes.

```javascript 
test("if an association record is deleted, it is removed from the association", function() {
  store.load(Group, { id: 1, people: [ 1 ] });
  store.loadMany(Person, [{ id: 1 }]);

  var group = store.find(Group, 1);
  var people = get(group, 'people');
  var person1 = store.find(Person, 1);

  equal( get(people,'length'), 1, "the record exists in the association");

  person1.deleteRecord();

  equal( get(people,'length'), 0, "the record is removed from the association");
});

```


However, if the record is created, added to the association and then deleted, it doesn't get removed from the association.  This is shown in the following test which currently fails.

```javascript
test("if an association record is created and then deleted, it is removed from the association", function() {
  store.load(Group, { id: 1, people: [] });
  var group = store.find(Group, 1);
  var people = get(group, 'people');
  var person = store.createRecord(Person, {});

  equal( get(people,'length'), 0, "there are no associations");

  people.pushObject(person);

  equal( get(people,'length'), 1, "the record is added to the association");

  person.deleteRecord();

  equal( get(people,'length'), 0, "the record is removed from the association");
});

```