Remove dead code

And adjust the affected test method by simplifying it.

There is no need to overwrite to_s Ruby method to make the only
test that uses the has_issue? method work. I mean, it works fine
using issue.title.

qa/qa/page/project/issue/index.rb

@@ -38,7 +38,7 @@ module QA
           end
 
           def has_issue?(issue)
-            has_element? :issue, issue_title: issue.to_s
+            has_element? :issue, issue_title: issue.title
           end
         end
       end

qa/qa/resource/issue.rb

@@ -38,10 +38,6 @@ module QA
         end
       end
 
-      def to_s
-        @title
-      end
-
       def api_get_path
         "/projects/#{project.id}/issues/#{id}"
       end