Remove ControllerContentModelAliasDeprecation for deprecatingAlias

So, this is kind of a fun one.

From what I can tell, `ControllerContentModelAliasDeprecation` was
introduced, due to Controllers having a `content` property, which
`model` was aliased to.
This was done because `ObjectController` and `ArrayController`
were proxying controllers.

But, in Ember, routes set up the `model` property of its respective
controller.
What this meant is that if someone declared a `content` property
in their controller, `model` would—seemingly unrelated—break,
hence the `ControllerContentModelAliasDeprecation` mixin.

Nowadays it is fine to override `content` because the source of
truth was reverse, and `model` is now the primary property, being
`content` the alias.
It is, then, time to remove the old deprecation and instead
deprecate the alias itself, so that in the future users can define
a `content` property in their controllers if they so desire.

Another API bites the dust.
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 '../computed/computed_macros';
 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: '2.17.0',
+    url: 'https://emberjs.com/deprecations/v2.x/#toc_controller-content-alias'
+  })
 });

packages/ember-runtime/tests/controllers/controller_test.js

@@ -108,7 +108,9 @@ QUnit.test('`model` is aliased as `content`', function() {
     model: 'foo-bar'
   }).create();
 
-  equal(controller.get('content'), 'foo-bar', 'content is an alias of model');
+  expectDeprecation(function () {
+    equal(controller.get('content'), 'foo-bar', 'content is an alias of model');
+  });
 });
 
 QUnit.test('`content` is moved to `model` when `model` is unset', function() {