kIndex_8_SkColorType is gone, so remove reference to it

Bug: 
Change-Id: I0360b904c54ae5fc0c42a9df46e41bfd5fb3f92b
Reviewed-on: https://chromium-review.googlesource.com/561876Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Mike Reed <reed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486554}

skia/config/SkUserConfig.h

@@ -208,10 +208,6 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #   define SK_USE_LEGACY_DISTANCE_FIELDS
 #endif
 
-#ifndef SK_SUPPORT_LEGACY_INDEX_8_COLORTYPE
-#define SK_SUPPORT_LEGACY_INDEX_8_COLORTYPE
-#endif
-
 #ifndef SK_DISABLE_DEFERRED_PROXIES
 #define SK_DISABLE_DEFERRED_PROXIES
 #endif

skia/public/interfaces/bitmap_skbitmap_struct_traits.cc

@@ -75,7 +75,6 @@ skia::mojom::ColorType SkColorTypeToMojo(SkColorType type) {
     case kGray_8_SkColorType:
       return skia::mojom::ColorType::GRAY_8;
     case kRGBA_F16_SkColorType:
-    case kIndex_8_SkColorType:
       // these are unsupported
       break;
   }