Optimize ClearNonLiveReferences: do not compact prototype transitions in GC.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index c82d0cf934ddfbb14fc44ce9765e2208bbeb9947..9232d0e69aaf9b753f7a31df0e33cf7a954e69ee 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -656,7 +656,6 @@ class MarkCompactCollector {
   // Map transitions from a live map to a dead map must be killed.
   // We replace them with a null descriptor, with the same key.
   void ClearNonLiveReferences();
-  void ClearNonLivePrototypeTransitions(Map* map);
   void ClearNonLiveMapTransitions(Map* map);
   void ClearMapTransitions(Map* map, Map* dead_transition);
   bool ClearMapBackPointer(Map* map);