trim out unneeded references to SkString

Change-Id: I01259b685859e2d95e4b87c0f611ccc074bd5229
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254807
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Reed <reed@google.com>
8 files changed