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 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 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.
Check for Rails.env instead of Rails
`Rails` constant is added by rails-html-sanitizer leading to bugs in
non-Rails apps using ActiveRecord and ActionMailer
Merge pull request #18196 from PavelPenkov/rails_env_instead_of_rails

Check for Rails.env instead of Rails
- Changed `target` to `target_reflection` to avoid warning `possible reference to past scope`
Add example to get Errored test to pass in Testing guide [ci skip]
targeting 2.2
Remove unnecessary double space
Merge pull request #18194 from claudiob/remove-action-controller-model-naming

Remove ActionController::ModelNaming
Merge pull request #18204 from chocoby/remove-double-space

Remove unnecessary double space
Merge pull request #18203 from kyledecot/ruby-2.2

Targeting 2.2
Merge pull request #18197 from vipulnsward/fix-ar-2-2warning

Fixed AR warning on ruby 2.2
- Changed `paths` to `routing_paths` to avoid warning `possible reference to past scope`
Merge pull request #18205 from vipulnsward/fix-railties-warning

Fixed warning `possible reference to past scope` from railties