This reverts commit 802c7b3e.

Reason for revert: Cluserfuzz found a reason to have this.

Original change's description:
> [string] DCHECK instead of test that ThinString's actuals have same length
>
> ThinStrings always forward to internalized strings that have the same
> character contents and thus the same length.
>
> Change-Id: I5929d266f96b23029f4786baf993a431cf4ad38d
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3541522
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Shu-yu Guo <syg@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#79582}

Change-Id: Ib8a7b962242f8b8ab3ffc5c70ed1c84d187e5b8e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3548459
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#79599}