[heap] Remove unused field

Field isn't used in V8 anymore.

Change-Id: I564cfb30250e45cface0b6bdfd1390a458385bac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2922243
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74821}

src/heap/heap.h

@@ -2106,8 +2106,6 @@ class Heap {
   std::vector<WeakArrayList> FindAllRetainedMaps();
   MemoryMeasurement* memory_measurement() { return memory_measurement_.get(); }
 
-  // The amount of memory that has been freed concurrently.
-  std::atomic<uintptr_t> external_memory_concurrently_freed_{0};
   ExternalMemoryAccounting external_memory_;
 
   // This can be calculated directly from a pointer to the heap; however, it is