This reverts commit 76b53b66.

Reason for revert: Can't be used easily in embedder (Chromium).

Original change's description:
> [api] Add a SharedArrayBuffersEnabled callback.
> 
> - Adds a SharedArrayBuffersEnabled callback and uses it to
>   enable/disable SABs per context. The feature flag is used
>   if no callback is registered.
> 
> Bug: chromium:923807
> Change-Id: I4d3472fcd79b158cb50dc98793aece6dbbb81d93
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2316901
> Reviewed-by: Adam Klein <adamk@chromium.org>
> Commit-Queue: Bill Budge <bbudge@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#69057}

TBR=bbudge@chromium.org,adamk@chromium.org

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

Bug: chromium:923807
Change-Id: I6e3ddfa9cd64ba3106b4a75ea7a5185f873facc9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2326952Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69134}