Remove SkPM4f::toF16

Android use of this API has been removed

Bug: skia:
Change-Id: I7f05704472a6024c9009123a169a8ece37761447
Reviewed-on: https://skia-review.googlesource.com/156361
Auto-Submit: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed