[heap] Remove left-over object stats tracking declaration.

R=mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1583323005

Cr-Commit-Position: refs/heads/master@{#33335}

src/heap/mark-compact.cc

@@ -1224,9 +1224,6 @@ class MarkCompactMarkingVisitor
   }
 
  private:
-  template <int id>
-  static inline void TrackObjectStatsAndVisit(Map* map, HeapObject* obj);
-
   // Code flushing support.
 
   static const int kRegExpCodeThreshold = 5;