This reverts commit ac2fb66b.

Reason for revert: Flakily crashes on several bots:
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Win32/18524
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Win64%20-%20msvc/6824
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Linux64%20-%20internal%20snapshot/19766

Original change's description:
> [wasm] Remove finisher task
> 
> This removes the finisher task and instead finishes compilation units
> from the background.
> It also changes ownership of the AsyncCompileJob to be shared among all
> tasks that still operate on it. The AsyncCompileJob dies when the last
> reference dies.
> 
> R=​ahaas@chromium.org
> CC=​​​mstarzinger@chromium.org
> 
> Bug: v8:7921, v8:8423
> Change-Id: Id09378327dfc146459ef41bc97176a8716756ae4
> Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
> Reviewed-on: https://chromium-review.googlesource.com/c/1335553
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#58630}

TBR=ahaas@chromium.org,clemensh@chromium.org

Change-Id: I6b332b66adaec8f713fb31f4c8517cae7ebb4645
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7921, v8:8423
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Reviewed-on: https://chromium-review.googlesource.com/c/1400420Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58634}