We are using `Ember.ArrayProxy` which includes `Ember.MutableArray` ([here](https://github.com/emberjs/ember.js/blob/beta/packages_es6/ember-runtime/lib/system/array_proxy.js#L62)).

`Ember.MutableArray` defines a `removeObject` function [here](https://github.com/emberjs/ember.js/blob/beta/packages_es6/ember-runtime/lib/mixins/mutable_array.js#L320).