Revert of Reland "[heap] Turn on parallel compaction" (patchset #1 id:1 of https://codereview.chromium.org/1444283002/ )

Reason for revert:
Looks like tasks are still scheduled during isolate shutdown after waiting for remaining tasks:
  https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20arm64%20-%20sim%20-%20MSAN/builds/5310/

Original issue's description:
> Reland "[heap] Turn on parallel compaction"
>
> This reverts commit e6820480.
>
> BUG=chromium:524425
> LOG=N
> CQ_EXTRA_TRYBOTS=tryserver.v8:v8_linux_arm64_gc_stress_dbg;tryserver.v8:v8_linux_gc_stress_dbg;tryserver.v8:v8_mac_gc_stress_dbg;tryserver.v8:v8_linux64_msan_rel;tryserver.v8:v8_linux64_tsan_rel
>
> Committed: https://crrev.com/c82d2323ffd5c399ba37da389ba0cdc72f46c97f
> Cr-Commit-Position: refs/heads/master@{#32009}

TBR=hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:524425

Review URL: https://codereview.chromium.org/1445093002

Cr-Commit-Position: refs/heads/master@{#32010}