Remove ControllerContentModelAliasDeprecation for deprecatingAlias

So, this is kind of a fun one.

ControllerContentModelAliasDeprecation was introduced, from what I
could tell, because in the 1.x days of Ember, Controllers had a
`content` property, which `model` was aliased to.
We decided to flip that, and make `model` the source of truth and
`content` the alias.
Problem though, until that flip, if someone overrode `content`,
`model` would break too, hence the addon.

Nowadays, it is now fine to override `content`, and we don't need
to warn anymore that it is deprecated. It is then, time to remove
it. Which I did. Removing code is the best.

We now bumped the deprecation up a notch and made it a deprecating
alias instead, and eventually we will remove the `content` alias.

Here's to removing yet more code in the future, and make Ember
sparkle-clean.

features.json

@@ -35,10 +35,10 @@
     "ember-views.render-double-modify": "2.0.0",
     "ember-routing.router-resource": "2.0.0",
     "ember-routing.top-level-render-helper": "2.11.0",
+    "ember-runtime.controller-content": "2.16.0",
     "ember-runtime.controller-proxy": "2.0.0",
     "ember-runtime.action-handler-_actions": "2.0.0",
     "ember-runtime.enumerable-contains": "2.7.0",
-    "ember-runtime.will-merge-mixin": "2.0.0",
     "ember-runtime.frozen-copy": "2.0.0",
     "ember-runtime.freezable-init": "2.0.0",
     "ember-string-utils.fmt": "2.0.0",

packages/ember-runtime/lib/mixins/controller.js

@@ -1,14 +1,14 @@
 import { Mixin, alias } from 'ember-metal';
+import { deprecatingAlias } from 'ember-runtime';
 import ActionHandler from './action_handler';
-import ControllerContentModelAliasDeprecation from './controller_content_model_alias_deprecation';
 
 /**
   @class ControllerMixin
   @namespace Ember
   @uses Ember.ActionHandler
   @private
 */
-export default Mixin.create(ActionHandler, ControllerContentModelAliasDeprecation, {
+export default Mixin.create(ActionHandler, {
   /* ducktype as a controller */
   isController: true,
 
@@ -38,12 +38,15 @@ export default Mixin.create(ActionHandler, ControllerContentModelAliasDeprecatio
 
     @property model
     @public
-   */
+  */
   model: null,
 
   /**
     @private
   */
-  content: alias('model')
-
+  content: deprecatingAlias('model', {
+    id: 'ember-runtime.controller.content-alias',
+    until: '2.16.0',
+    url: 'https://emberjs.com/deprecations/v2.x#toc_controller-content-alias'
+  })
 });