remove dead code associated with SK_SUPPORT_LEGACY_RASTERPIPELINE

Now we just look at the paint to decide if the user wants to dither.

Needs this to land first:
https://skia-review.googlesource.com/c/19448/

Bug: skia:
Change-Id: I31940696e114922bcc8fb9ca3c24c2eb0a622800
Reviewed-on: https://skia-review.googlesource.com/19496
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed