Remove legacy avatar code

- Remove method that was only left around because the
  [api](https://github.com/discourse/discourse_api/pull/53) called it
- Modify test to use new route instead of legacy route

https://meta.discourse.org/t/legacy-route-for-avatars/22838/2

spec/controllers/users_controller_spec.rb

@@ -1218,7 +1218,7 @@ describe UsersController do
             upload = Fabricate(:upload)
             Upload.expects(:create_for).returns(upload)
             # enqueues the user_image generator job
-            xhr :post, :upload_avatar, username: user.username, file: user_image_url, image_type: "avatar"
+            xhr :post, :upload_user_image, username: user.username, file: user_image_url, image_type: "avatar"
             json = JSON.parse(response.body)
             json['url'].should == "/uploads/default/1/1234567890123456.png"
             json['width'].should == 100