SkString: remove externally unused ::setUTF16() from API

Moved to SkStringUtils in src/

Change-Id: I026e3a325570bbf34e90797d921cb2f05b9a29f6
Reviewed-on: https://skia-review.googlesource.com/111602
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
8 files changed