Remove the remaining parts of background idle notification.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 3be440984f391bdc1bcfa0a74469ef46f99c95e9..942087e060921bb734827b6af0a6df0eb10706e6 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1801,7 +1801,6 @@ class Heap {
   void IdleNotificationEpilogue(GCIdleTimeAction action,
                                 GCIdleTimeHeapState heap_state, double start_ms,
                                 double deadline_in_ms);
-  void CheckBackgroundIdleNotification(double idle_time_in_ms, double now_ms);
 
   inline void UpdateAllocationsHash(HeapObject* object);
   inline void UpdateAllocationsHash(uint32_t value);