This reverts commit 7a0ae73b.

Reason for revert: Not useful after all, no tests, we can reland if we
do end up needing it.

Original change's description:
> [ast] Add a Flatten method for AstConsString
> 
> This allows off-thread flattening.
> 
> Bug: chromium:1011762
> Change-Id: If83f7bbcbf74165987a4c157184f5b92dc554971
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1924437
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Auto-Submit: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#65075}

TBR=leszeks@chromium.org,verwaest@chromium.org

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

Bug: chromium:1011762
Change-Id: Ia72e5abdc9b6149a337565576806427dcd1d11c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1991484Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65638}