fix(library/mt_task_queue): reverse dependencies may already be queued

When we check whether the dependencies for a task have already been
evaluated and then accordingly move the task from waiting to queued, we
do not remove it from the reverse dependency lists it appears in.

src/library/mt_task_queue.cpp

@@ -125,6 +125,7 @@ void mt_task_queue::spawn_worker() {
                                         enqueue(rdep);
                                 }
                                 break;
+                            case task_result_state::QUEUED: break;
                             case task_result_state::FAILED: break;
                             default:
                                 lean_unreachable();