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.

spec/routing/polymorphic_routes_spec.rb

@@ -53,12 +53,6 @@ describe "Polymorphic routes" do
       )
     end
 
-    it "routes poll questions" do
-      question = create(:poll_question)
-
-      expect(polymorphic_path(question)).to eq question_path(question)
-    end
-
     it "routes topics" do
       community = create(:proposal).community
       topic = create(:topic, community: community)