Remove dithering during SkConvertPixels

Now that https://chromium-review.googlesource.com/c/chromium/src/+/1495208
has landed, this is safe to land.

Bug: b/124403051 skia:8787
Change-Id: I087cd2313343229176a1814c2097dbe8f77620cc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/195886
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>