Remove GrAAType::kMixedSamples

Our only use case for mixed samples is stencil-then-cover. This mode
is now handled by AATypeFlags::kMixedSampledStencilThenCover.

Bug: skia:
Change-Id: Id7431cf83ccb20752d1bc85c6ad41efe408e0359
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/200841
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
3 files changed