Remove obsolete "pending" test

I'd say this feature is actually tested in the "proposal polls specific
validations"; the empty test was probably added by accident in commit
4b8cc85c4.

spec/models/poll/poll_spec.rb

@@ -35,8 +35,6 @@ describe Poll do
       poll.ends_at = 2.months.ago
       expect(poll).not_to be_valid
     end
-
-    pending "no overlapping polls for proposal polls are allowed"
   end
 
   describe "proposal polls specific validations" do