minor Sk4px cleanup

I've removed a bunch of unused code,
inlined routines that are only used once,
removed platform code where a portable code works fine,
and renamed methods and updated comments.

I'm thinking about getting rid of Sk4px entirely, and boiling it down
instead to a couple helper methods on Sk16b/Sk16h.  This is a start.

Change-Id: I35b6c44710aa2cefe8c1e07fc2cb877a4042e8a5
Reviewed-on: https://skia-review.googlesource.com/c/178985
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Herb Derby <herb@google.com>
9 files changed