Hide "Maximum number of votes" message for unique and open-ended questions

The "Maximum number of votes" text in poll question show was unnecessary.
It appeared for both unique and open-ended questions, but it only makes
sense for questions that allow multiple answers.

app/views/admin/poll/questions/show.html.erb

@@ -35,7 +35,7 @@
         <br>
         <%= VotationType.human_attribute_name("vote_type.#{@question.vote_type}") %>
       </p>
-      <% if @question.max_votes.present? %>
+      <% if @question.multiple? %>
         <p>
           <strong><%= VotationType.human_attribute_name("max_votes") %></strong>
           <br>

spec/system/admin/poll/questions_spec.rb

@@ -88,6 +88,7 @@ describe "Admin poll questions", :admin do
 
         expect(page).to have_content "Question with unique answer"
         expect(page).to have_content "Unique answer"
+        expect(page).not_to have_content "Maximum number of votes"
       end
 
       scenario "Multiple" do
@@ -104,6 +105,7 @@ describe "Admin poll questions", :admin do
 
         expect(page).to have_content "Question with multiple answers"
         expect(page).to have_content "Multiple answers"
+        expect(page).to have_text "Maximum number of votes 6", normalize_ws: true
       end
 
       scenario "Open-ended" do
@@ -119,6 +121,7 @@ describe "Admin poll questions", :admin do
 
         expect(page).to have_content "What do you want?"
         expect(page).to have_content "Open-ended"
+        expect(page).not_to have_content "Maximum number of votes"
       end
     end
   end