The "length" field of strings should never be mutated once it has
been initialized. This means that the checks done by StringRef::length
were never really useful. This CL thus removes them.

Bug: chromium:1352386
Change-Id: I49f681daad119553eb0d4f1c2315bff5138197d7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3829541
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82536}