Also we're not creating these any more via StringUtil in blink,
so these two helpers can go.

Change-Id: Ifdf700171ebc46cfbd6ce0af9a6f6ba73419a5a8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1887689
Auto-Submit: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64626}