Delete GradientShaderCache

Dead code.

Change-Id: If415e603b8e898da72ad95f32596445900794d23
Reviewed-on: https://skia-review.googlesource.com/63562
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
6 files changed