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/WorkItem.cpp

@@ -89,6 +89,14 @@ WorkItem::Deinit()
 	}
 }
 
+void
+WorkItem::ScheduleClear()
+{
+	if (_wq != nullptr) {
+		_wq->Remove(this);
+	}
+}
+
 float
 WorkItem::elapsed_time() const
 {