Remove ActionController::ModelNaming

The methods in these modules are not used anywhere. They used to be
invoked in polymorphic_routes.rb but their usage was removed in e8210450.

What is your opinion about removing these methods?

They do belong to the public API, but in reality their code has already been duplicated to ActionView::ModelNaming, since they are used by methods like `dom_id` and `dom_class` to associated records with DOM elements (in
ActionView).

Please tell me if you think that removing this module is a good idea and,
in that case, if the PR is okay as it is, or you'd rather start by showing
a deprecation message, and remove the module in Rails 5.1.

actionpack/lib/action_controller/model_naming.rb

-module ActionController
-  module ModelNaming
-    # Converts the given object to an ActiveModel compliant one.
-    def convert_to_model(object)
-      object.respond_to?(:to_model) ? object.to_model : object
-    end
-
-    def model_name_from_record_or_class(record_or_class)
-      convert_to_model(record_or_class).model_name
-    end
-  end
-end

actionpack/lib/action_dispatch/routing/polymorphic_routes.rb

-require 'action_controller/model_naming'
-
 module ActionDispatch
   module Routing
     # Polymorphic URL helpers are methods for smart resolution to a named route call when
@@ -55,8 +53,6 @@ module Routing
     #   form_for([blog, @post])         # => "/blog/posts/1"
     #
     module PolymorphicRoutes
-      include ActionController::ModelNaming
-
       # Constructs a call to a named RESTful route for the given record and returns the
       # resulting URL string. For example:
       #