Removes:
  * ReturnValue::Set(Persistent)
  * String::NewFromUtf8/NewFromTwoByte/NewExternal overloads that
  returned Locals instead of MaybeLocal
  * String::NewStringType

Now that the old String overloads are gone, the new ones can now have
a default parameter for NewStringType matching the old overloads.

Bug: v8:7289, v8:7281, v8:9183
Change-Id: If66e6d587ac778e015c281b376a9b4d6093f6ec3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1591605Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61373}