Remove GradientShaderCache dither and paint alpha support

The gradient cache is only used for complex GPU gradients, and is
always initialized with no dithering and opaque alpha.

Remove the (now-dead) code supporing ditherning and non-opaque alpha.

Change-Id: Iad694fa760e9a236efadd4cf92fbc7f8bcacadbe
Reviewed-on: https://skia-review.googlesource.com/63520
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed