If we cancel the task in the thread where it's supposed to run,
task cancelling will always succeed.

This simplifies the logic.

Bug: v8:10239
Change-Id: I3fb5c93a49c52d958aa947d693700161bc18eee5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2332807Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69203}