The value of the counter is never used, so remove it. This not only
cleans up code, but also makes triggering tier-up a little bit faster
because we do not need to take a mutex any more.

R=ahaas@chromium.org

Bug: v8:12899
Change-Id: I09f7df4d49b0d94567cd84ccb2aaa6fd86b96c37
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3667645Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80931}