Remove unused premul/unpremul FP code

Bug: skia:
Change-Id: I911ee066ce4d0175cee3ee3868a86955d486687c
Reviewed-on: https://skia-review.googlesource.com/159060
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
9 files changed