ScheduledWorkItem::ScheduleClear: remove item from runnable queue

The existing behavior is unexpected: if the work item is already on the
runnable queue, it will still be triggered after a call to ScheduleClear().
This can lead to race conditions.

platforms/common/px4_work_queue/WorkQueue.cpp

@@ -160,6 +160,7 @@ WorkQueue::Run()
 			work_unlock(); // unlock work queue to run (item may requeue itself)
 			work->RunPreamble();
 			work->Run();
+			// Note: after Run() we cannot access work anymore, as it might have been deleted
 			work_lock(); // re-lock
 		}