remove c=128 src-mode optimization

Not that big an improvement for the code size cost.
We can do way better, e.g. with platform specific code.

Change-Id: I540a9b8759fa76f578eba8d6e56bc1156433b0fd
Reviewed-on: https://skia-review.googlesource.com/c/193968
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed