This reverts commit 56a6fda3.

Reason for revert: Makes tsan flaky:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20TSAN/builds/15038

Original change's description:
> [parser] Inital parallel parse tasks implementation.
> 
> While parsing top-level code eager functions are skipped just like lazy
> ones, but also a parse task is created for each.
> 
> The parse tasks are run by the compiler dispatcher and can be executed
> either on background thread or in idle time.
> After parsing of top-level code finishes it waits for all unfinished
> parser tasks - possibly picking up and executing them on current thread.
> Afterwards parse task results are stitched together with top-level AST,
> in case of failures eager functions are treated just like lazy -
> parsing/compilation is retriggered for them in the runtime and proper
> errors are generated (performance is not optimized for error case at
> all).
> 
> BUG=v8:6093
> 
> Change-Id: I718dd2acc8a70ae1b09c2dea2616716605d7b05d
> Reviewed-on: https://chromium-review.googlesource.com/483439
> Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Reviewed-by: Jochen Eisinger <jochen@chromium.org>
> Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#44849}

TBR=marja@chromium.org,vogelheim@chromium.org,jochen@chromium.org,wiktorg@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:6093

Change-Id: I17e689efee7d216d28a94a5c8147022ae7e830dd
Reviewed-on: https://chromium-review.googlesource.com/486883Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44859}