This reverts commit 902f48bd.

Reason for revert: Made TSAN unhappy: https://ci.chromium.org/p/v8/builders/ci/V8%20Linux64%20TSAN%20-%20isolates/9480

Original change's description:
> [wasm][debug] Fix tier down for multiple isolates
> 
> If multiple isolates are using the same module, we need to keep it
> tiered down as long as any isolate still has a debugger open.
> Also, we cannot short-cut the {NativeModule::TierDown} method, since the
> previously triggered tier down might not have finished yet.
> For now, each isolate starts an independent tier down (i.e. a full
> recompilation). We could optimize this later by skipping functions that
> are already tiered down, or are already scheduled for tier down, but we
> still need to wait for tier-down to finish on each isolate.
> 
> R=​thibaudm@chromium.org
> 
> Bug: v8:10359
> Change-Id: I7ea6a6f5d3977e48718ac5bc94f9831541f6173f
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190758
> Commit-Queue: Clemens Backes <clemensb@chromium.org>
> Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#67716}

TBR=clemensb@chromium.org,thibaudm@chromium.org

Change-Id: Ibf650e8b6143471b44f2822c1737e7de5f8bdb20
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:10359
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2194372Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67720}