[heap] Ceanup: Remove weak cell scavenging code

WeakCells are never allocated in new space.

BUG=chromium:651354

Review-Url: https://codereview.chromium.org/2476323003
Cr-Commit-Position: refs/heads/master@{#40841}

src/heap/heap.cc

@@ -1672,8 +1672,6 @@ void Heap::Scavenge() {
     TRACE_GC(tracer(), GCTracer::Scope::SCAVENGER_WEAK);
     // Copy objects reachable from the encountered weak collections list.
     scavenge_visitor.VisitPointer(&encountered_weak_collections_);
-    // Copy objects reachable from the encountered weak cells.
-    scavenge_visitor.VisitPointer(&encountered_weak_cells_);
   }
 
   {