StressConcurrentAllocatorTask now checks whether tear down was started
for the isolate to avoid allocation failures.

As a drive-by change remove the unused method
ConcurrentAllocator::PerformCollectionAndAllocateAgain.

Bug: v8:10315
Change-Id: Iba329ebbd782e9f8f11e9b8ec644bf28ab9c80ab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2423703Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70044}