There is no reason to allocate the vectors on the heap. Their
content will be heap-allocated anyway, and they are cheap to move
around.

Drive-by: Remove an unused counter.

R=thibaudm@chromium.org

Bug: v8:11164
Change-Id: I5660ecf5db7e8915a27255bae0215d5368c7d10e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2644937
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72291}