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/1028337Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52795}

src/elements.cc

@@ -2298,9 +2298,6 @@ class FastElementsAccessor : public ElementsAccessorBase<Subclass, KindTraits> {
       if (IsDoubleElementsKind(KindTraits::Kind)) {
         MemMove(dst_elms->data_start() + dst_index,
                 dst_elms->data_start() + src_index, len * kDoubleSize);
-      } else if (IsSmiElementsKind(KindTraits::Kind)) {
-        MemMove(dst_elms->data_start() + dst_index,
-                dst_elms->data_start() + src_index, len * kPointerSize);
       } else {
         DisallowHeapAllocation no_gc;
         heap->MoveElements(FixedArray::cast(*dst_elms), dst_index, src_index,