Revert of "[heap] Add more tasks for parallel compaction" (patchset #4 id:100001 of https://codereview.chromium.org/1365743003/ )

Reason for revert:
failing again: https://chromegw.corp.google.com/i/client.v8/builders/V8%20Mac/builds/4505/steps/Mozilla%20%28flakes%29/logs/regress-416628

Original issue's description:
> Reland of "[heap] Add more tasks for parallel compaction"
>
> - We now compute the number of parallel compaction tasks, depending on the
>   evacuation candidate list, the number of cores, and some hard limit.
> - Free memory is moved over to compaction tasks (up to some limit)
> - Moving over memory is done by dividing the free list of a given space up among
>   other free lists. Since this is potentially slow we limit the maximum amount
>   of moved memory.
>
> This reverts commit bfccd518.
>
> BUG=chromium:524425
> LOG=N
>
> Committed: https://crrev.com/7e283d746a194ceaaca114e2ba17504653d6a109
> Cr-Commit-Position: refs/heads/master@{#30945}

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

Review URL: https://codereview.chromium.org/1371653002

Cr-Commit-Position: refs/heads/master@{#30947}