Remove another GrColor helper function

GrPremulColor is only use in test code, and is redundant

Bug: skia:
Change-Id: I4a22941d28cbec3eb203bca393cfeccffe04e053
Reviewed-on: https://skia-review.googlesource.com/c/165524
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
2 files changed