remove unused SkBitmap::copyPixelsTo

Needs https://codereview.chromium.org/2812853002/ to land first

Bug: skia:6465
Change-Id: I531e33b2848cd995f20844786ed1a8d34d63fb64
Reviewed-on: https://skia-review.googlesource.com/13171
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed