Delete SkARGBImageEncoder.

It appears to be unused now except by its unit test, also deleted.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2208433002

No public API changes.
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/2208433002
4 files changed