We don't use StatsCounters anymore for heap work, and this particular
counter causes issues for OffThreadSpace, as it can trigger a
non-thread-safe counter callback.

We could instead make this a thread-safe counter, but since it's unused
we may as well just remove it entirely.

Bug: chromium:1011762
Change-Id: I5af5ec5c408691ebfb762d87334ec4af54b1c0e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2126914
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67340}