Redundant and confusing code in UserController#current_user

I believe https://github.com/gothinkster/elixir-phoenix-realworld-example-app/blob/master/lib/real_world_web/controllers/user_controller.ex#L29-L35 is redundant and confusing. There is no need to check `user` for nil and render 404.json, as Guardian already has an error handler which will be invoked when trying to access `UserController#current_user` without being authenticated.