Remove unused SkZeroShaderContext

Change-Id: I3e9d9cb9c2f948b0bb363871e038ade1cf2f3d67
Reviewed-on: https://skia-review.googlesource.com/17308
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed