Remove duplicated code

Author: rm-openai

src/agents/result.py

@@ -216,5 +216,3 @@ def _cleanup_tasks(self):
 
         if self._output_guardrails_task and not self._output_guardrails_task.done():
             self._output_guardrails_task.cancel()
-            self._output_guardrails_task.cancel()
-            self._output_guardrails_task.cancel()