Since https://crrev.com/c/3806439 on-stack traced handles are marked
conservatively when being used in combination with CppHeap.

This change removes the precise on-stack representation of the
internal traced nodes as they nodes would anyways be marked
conservatively. The effects are:
- cheaper representation (just a single node space);
- uniform handling: no checks to distinguish on-stack vs on-heap;
- no brittleness around cleaning on-stack handles when the event loop
 is empty;

Change-Id: Id859623bfed77a66bdd064ea8065536264515eae
Bug: v8:13141
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3812039Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82306}