Remove unused code dealing with unaccepted_responses

These unaccepted_responses were only used by Smartdown flows, and we removed the
final parts of Smartdown in d2138cd.

There's definitely more code that can be removed now that Smartdown has gone but
I figured I'd do this as I recently came across it.

Author: chrisroos

app/controllers/smart_answers_controller.rb

@@ -86,11 +86,6 @@ def redirect_response_to_canonical_url
         responses: @presenter.current_state.responses,
         protocol:  (request.ssl? || Rails.env.production?) ? 'https' : 'http',
       }
-      if @presenter.current_state.unaccepted_responses
-        @presenter.current_state.unaccepted_responses.each_with_index do |unaccepted_response, index|
-          redirect_params["previous_response_#{index+1}".to_sym] = unaccepted_response.to_s
-        end
-      end
       redirect_to redirect_params
     end
   end