remove unused getAdvance from SkScalerContext

Change-Id: Ia660454c30535bd18535a71253176ee0276875cf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334098
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed