This reverts commit 8aa6b15f.

Reason for revert: broke TSAN https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux64%20TSAN%20-%20stress-incremental-marking/1497/overview

Original change's description:
> Disable bytecode flushing once we toggle coverage mode.
>
> Changing coverage mode generated different bytecode in some cases.
> Hence it is not safe to flush bytecode once we toggle coverage mode.
>
> Bug: chromium:1147917
> Change-Id: I9e640aeaec664d3d4a4aaedf809c568e9ad924fc
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2615020
> Commit-Queue: Mythri Alle <mythria@chromium.org>
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#71985}

TBR=rmcilroy@chromium.org,mythria@chromium.org

Change-Id: Id4c95da337e291437b7856e2fe7004e1e6405515
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1147917
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2619402Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71989}