add private

In my application, I'm including Jpmobile::ViewSelector at ApplicationController.
And I'm just refactoring my appliation and using [traceroute gem](https://github.com/amatsuda/traceroute) to pickup unused action.

$rake traceroute

Unused routes:
  rails/info#routes
  rails/info#index
  rails/welcome#index
Unreachable action methods:
  users#show
  users#register_mobile
  users#disable_mobile_view!

I don't want to display #register_mobile,#disable_mobile_view!
please merge this pull request.