Revert of [wasm] Implement parallel compilation. (patchset #6 id:100001 of https://codereview.chromium.org/1961973002/ )

Reason for revert:
The ThreadSanitizer finds data races.

Original issue's description:
> [wasm] Implement parallel compilation.
>
> With this CL it is possible to compile a wasm module with multiple
> threads in parallel. Parallel compilation works as follows:
>
> 1)   The main thread allocates a compilation unit for each wasm function.
> 2)   The main thread spawns WasmCompilationTasks which run on the
>      background threads.
> 3.a) The background threads and the main thread pick one compilation unit
>      at a time and execute the parallel phase of the compilation unit.
>      After finishing the execution of the parallel phase, the compilation
>      unit is stored in a result queue.
> 3.b) If the result queue contains a compilation unit, the main thread
>      dequeues it and finishes its compilation.
> 4)   After the execution of the parallel phase of all compilation units has
>      started, the main thread waits for all WasmCompilationTasks to finish.
> 5)   The main thread finalizes the compilation of the module.
>
> I'm going to add some additional tests before committing this CL.
>
> R=titzer@chromium.org, bmeurer@chromium.org, mlippautz@chromium.org, mstarzinger@chromium.org
>
> Committed: https://crrev.com/17215438659d8ff2d7d55f95226bf8a1477ccd79
> Cr-Commit-Position: refs/heads/master@{#36178}

TBR=bmeurer@chromium.org,mlippautz@chromium.org,mstarzinger@chromium.org,titzer@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.chromium.org/1965243003
Cr-Commit-Position: refs/heads/master@{#36182}