Since we always flatten the string upfront, we don't need to
implement the the UTF8 conversion as a string visitor anymore.

R=petermarshall@chromium.org

Bug: v8:6780, v8:8605
Change-Id: I27946551d7c3742f47ac36d5c909c19a7f2b0371
Reviewed-on: https://chromium-review.googlesource.com/c/1371828
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58312}