Avoid crash when removing job which might not exist in async_.py

I found a scenario once though harder to reproduce that scheduler crashes when a job runs longer than the schedule which has an interval of say 1 minute. I believe the duration is lesser important, but it is possible that the job runs longer than 1 minute and when the time comes to remove it then it do not exist and calling python `set.remove(id)` crashes the scheduler which should not be so. Hence protecting it.

Author: neeraj9

src/apscheduler/_schedulers/async_.py

@@ -1216,4 +1216,4 @@ async def _run_job(self, job: Job, func: Callable[..., Any], executor: str) -> N
             finally:
                 current_job.reset(token)
         finally:
-            self._running_jobs.remove(job)
+            self._running_jobs.discard(job)