This simplifies the logic since we can guarantee to have a
Handle<String>. The removed constructor was only used in tests.

Change-Id: I13519e474fe92892e9e8a39802d84cfab2c5b5ed
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2505711
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70849}