fix: Remove duplicate mark_task_as_started listener

Only one of the listeners was unsubscribed so they would pile up in
the WorkerEvent publisher.

Author: tpoliaw

src/blueapi/worker/task_worker.py

@@ -318,7 +318,6 @@ def mark_task_as_started(event: WorkerEvent, _: str | None) -> None:
         sub = self.worker_events.subscribe(mark_task_as_started)
         try:
             self._current_task_otel_context = get_current()
-            sub = self.worker_events.subscribe(mark_task_as_started)
             """ Cache the current trace context as the one for this task id """
             self._task_channel.put_nowait(trackable_task)
             task_started.wait(timeout=5.0)