Remove obsolete routes for poll questions

The routes for poll questions were accidentally deleted in commit
5bb831e959 when deleting the `:show` action, and restored in commit
9871503c5e. However, the deleted code was:

```
resources :questions, only: [:show], controller: 'polls/questions' (...)
```

While the restored code was:

```
resources :questions, controller: 'polls/questions' (...)
```

Meaning we forgot to add the `only: []` option when restoring the
routes.

We also forgot to remove the `before_action` code when deleting the
`:show` action, so we're removing it now.

config/routes/poll.rb

@@ -4,7 +4,7 @@ resources :polls, only: [:show, :index] do
     get :results
   end
 
-  resources :questions, controller: "polls/questions", shallow: true do
+  resources :questions, controller: "polls/questions", shallow: true, only: [] do
     post :answer, on: :member
     resources :answers, controller: "polls/answers", only: :destroy, shallow: false
   end