[heap] Remove unused PretenuringScope class forward declaration.

Bug: 
Change-Id: I28d6905a0c527a171deae89374ef54c2b4b97d3d
Reviewed-on: https://chromium-review.googlesource.com/722960Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48637}

src/heap/heap.h

@@ -1551,7 +1551,6 @@ class Heap {
 
  private:
   class SkipStoreBufferScope;
-  class PretenuringScope;
 
   typedef String* (*ExternalStringTableUpdaterCallback)(Heap* heap,
                                                         Object** pointer);