Remove kIndex_8_GrPixelConfig

It's been disabled for a long time (GPUs don't support it, and it actually
caused performance regression in testing).

BUG=skia:4333

Change-Id: I6e2bf755f765168fd616de6c9c023c6fbd5abd20
Reviewed-on: https://skia-review.googlesource.com/6897
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/vk/GrVkGpu.cpp b/src/gpu/vk/GrVkGpu.cpp
index f31c743..853e68c 100644
--- a/src/gpu/vk/GrVkGpu.cpp
+++ b/src/gpu/vk/GrVkGpu.cpp
@@ -257,7 +257,7 @@
 bool GrVkGpu::onGetWritePixelsInfo(GrSurface* dstSurface, int width, int height,
                                    GrPixelConfig srcConfig, DrawPreference* drawPreference,
                                    WritePixelTempDrawInfo* tempDrawInfo) {
-    if (kIndex_8_GrPixelConfig == srcConfig || GrPixelConfigIsCompressed(dstSurface->config())) {
+    if (GrPixelConfigIsCompressed(dstSurface->config())) {
         return false;
     }