Fix assertion error on delete with records that have hasMany relationships

See emberjs/data#2124.

Basically the issue is that because deleteRecord returns a hash with a modified _rev attribute,  the following assertion error is thrown if you attempt to delete a record that has hasMany relationships:
```
Error: Assertion Failed: calling set on destroyed object
    at new Error (native)
    at Error.Ember.Error (vendor/ember/ember.js:910:19)
    at Object.Ember.assert (vendor/ember/ember.js:73:11)
    at set (vendor/ember/ember.js:3029:9)
    at Ember.Object.extend.hasManyDidChange (vendor/ember-data/ember-data.js:5951:11)
    at eval (vendor/ember-data/ember-data.js:5940:18)
    at eval (vendor/ember/ember.js:3428:16)
    at Object.OrderedSet.forEach (vendor/ember/ember.js:3271:10)
    at Object.Map.forEach (vendor/ember/ember.js:3426:10)
    at Ember.Object.extend.reloadHasManys (vendor/ember-data/ember-data.js:5937:23) 
```

Is there really any reason/use case that deleteRecord needs to return the hash it currently does?