Revert of Use background tasks for the compiler dispatcher (patchset #5 id:80001 of https://codereview.chromium.org/2606263002/ )

Reason for revert:
Causes IgnitionCompilerDispatcherTest.FinishNowWithBackgroundTask to fail.

https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20noi18n%20-%20debug/builds/11209

Original issue's description:
> Use background tasks for the compiler dispatcher
>
> BUG=v8:5215
> R=marja@chromium.org,vogelheim@chromium.org
>
> Review-Url: https://codereview.chromium.org/2606263002
> Cr-Commit-Position: refs/heads/master@{#42035}
> Committed: https://chromium.googlesource.com/v8/v8/+/7a1b3a7bebbef88e72c9f7747e1930f10ee10c80

TBR=marja@chromium.org,vogelheim@chromium.org,jochen@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:5215

Review-Url: https://codereview.chromium.org/2614433002
Cr-Commit-Position: refs/heads/master@{#42037}