Now that all external pointers have been sandboxed,
V8_SANDBOXED_EXTERNAL_POINTERS is no longer needed. This change also
shrinks external pointer slots to 32 bits when the sandbox is enabled.

Bug: v8:10391
Change-Id: Iccbef27ac107b988cb23fe9ef66da6fe0bae087a
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3869269Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83083}