Remove ActionController::ModelNaming

The methods in these modules are not used anywhere. 
They used to be invoked in `polymorphic_routes.rb` but their last usage was removed in e821045.

What is your opinion about removing these methods?
They do belong to the public API, but in reality their code has [already been duplicated into `ActionView::ModelNaming`](https://github.com/rails/rails/blob/a2059068c02c25e61cc7d5701b0bbd2913e4012f/actionview/lib/action_view/model_naming.rb). 

The ActionView module is still used (by `dom_id` and `dom_class`to associate records with DOM elements).
The ActionController module is not.

Please tell me if you think that removing this module is a good idea and if you feel like the PR is okay as it is, or whether some deprecation warning should be displayed.