[graphite] Simplify PerEdgeAAQuadRenderStep frag shader and varyings.

* Remove unused cases in fragment shader
* Remove now unused varyings
* Change perPixelControl varying to single-valued coverageScale

Bug: b/307950537
Change-Id: I42b2fed8b617d103c9c72d3b1c790ce6f194b897
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/772658
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
7 files changed