This reverts commit 9a7c4bfe.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Revert "[elements] Improve Array.prototype.splice speed"
> 
> This reverts commit dcdabdc8.
> 
> Reason for revert: broke tsan.
> 
> Original change's description:
> > [elements] Improve Array.prototype.splice speed
> > 
> > By using memmove for SMI elements we get a roughly 3x speedup over the slower
> > iterative copying with write barriers.
> > 
> > Bug: chromium:835558
> > Change-Id: I73da07a1648a3495ff78212ffa1ed949d205a7d2
> > Reviewed-on: https://chromium-review.googlesource.com/1028236
> > Reviewed-by: Igor Sheludko <ishell@chromium.org>
> > Commit-Queue: Camillo Bruni <cbruni@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#52792}
> 
> TBR=cbruni@chromium.org,ishell@chromium.org
> 
> Change-Id: I77c46fe3d47d651de3c39df9fbf5f30c340188e2
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: chromium:835558
> Reviewed-on: https://chromium-review.googlesource.com/1028337
> Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
> Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52795}

TBR=kozyatinskiy@chromium.org,cbruni@chromium.org,ishell@chromium.org

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

Bug: chromium:835558
Change-Id: I57aedb3536b81c97cf4e7ab6d863aa1dc24c20b4
Reviewed-on: https://chromium-review.googlesource.com/1032743Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52857}