Remove SkUnPreMultiply::UnPreMultiplyPreservingByteOrder

Also remove the second, bespoke implementation in displacement
map effect.

Bug: skia:
Change-Id: I7363673337caba73c1311fe3cef4385dd7d1804e
Reviewed-on: https://skia-review.googlesource.com/c/167840
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed