Remove update:application_controller rake task.

This rake task was introduced in [9e08a3](https://github.com/rails/rails/commit/9e08a3bb1d47f79b6953056e72eee58e86d83ead) and was used to rename ```app/controllers/application.rb``` to ```app/controllers/application_controller.rb```.

This was used to migrate from Rails < 2.3 to Rails 2.3. I think this can be removed now.

I'm not sure if this needs changelog entry if anyone will decide to merge it.