This reverts commit c2e4183a.

Reason for revert: Uninitialized value on MSAN, see https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20arm64%20-%20sim%20-%20MSAN/26888

Original change's description:
> [runtime] Speed up String::IsOneByte
> 
> Check uintptr_t sized blocks of UTF16 chars at a time similar to NonAsciiStart.
> 
> Change-Id: Ib4c498cc064e6ac7b6f8cc87ab700eafc8132c39
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1622107
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#61693}

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

Change-Id: I2e18c9476f0ed47e77da87d2308a3429ef4f515c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1622112Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61697}