remove PremulLinear

No one is using it at the moment, and I'd prefer no
one ever stumble into it thinking it's a good idea.

Change-Id: I5db49252413329547f1d703756fd4d38c0fd727d
Reviewed-on: https://skia-review.googlesource.com/150545
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed