[heap] remove unnecessary std::atomic in PagedSpace::available_

Bug: v8:9329
Change-Id: Ia6592ff4d3046617fa536a6e2a9663b3dd73c5da
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1760809Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@google.com>
Cr-Commit-Position: refs/heads/master@{#63248}

src/heap/spaces.h

@@ -373,7 +373,7 @@ class FreeList {
   FreeListCategory** categories_ = nullptr;
 
   // |available_|: The number of bytes in this freelist.
-  std::atomic<size_t> available_{0};
+  size_t available_ = 0;
 
   friend class FreeListCategory;
   friend class Page;