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This reverts commit 22a95fed.

Reason for revert: class will not be used anymore, since this change:
https://crrev.com/c/2335639

Original change's description:
> [Extensions] Add ParallelUnpacker class
> 
> This class makes it easy to starts multiple SandboxedUnpacker's in
> parallel, and get notified as soon as any of them completes. Since
> SandboxedUnpacker is disk-bound and often takes >2s to run, running them
> in parallel can be beneficial.
> 
> Right now ParallelUnpacker is not used anywhere, but a follow-up CL will
> use it  to make extensions update in parallel, which will speed up
> extension installs when many of them are queued at the same time (such
> as for force-installed extensions).
> 
> Bug: 1103447
> Change-Id: Ibc35588134fe0e91c4f7a243677181a08c642552
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2294563
> Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
> Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#791835}

TBR=rdevlin.cronin@chromium.org,nicolaso@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1103447
Change-Id: Ibd1459dfdf6104ad5aac0bf4f62b5933ad437af1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367988Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801392}