Remove support for decoding to kIndex_8

Fix up callsites, and remove tests that no longer make sense.

Bug: skia:6828
Change-Id: I2548c4b7528b7b1be7412563156f27b52c9d4295
Reviewed-on: https://skia-review.googlesource.com/21664
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
diff --git a/src/codec/SkCodecImageGenerator.cpp b/src/codec/SkCodecImageGenerator.cpp
index 20d547a..57f7fb8 100644
--- a/src/codec/SkCodecImageGenerator.cpp
+++ b/src/codec/SkCodecImageGenerator.cpp
@@ -44,8 +44,7 @@
                                         const Options& opts) {
     SkCodec::Options codecOpts;
     codecOpts.fPremulBehavior = opts.fBehavior;
-    SkCodec::Result result = fCodec->getPixels(info, pixels, rowBytes, &codecOpts, nullptr,
-                                               nullptr);
+    SkCodec::Result result = fCodec->getPixels(info, pixels, rowBytes, &codecOpts);
     switch (result) {
         case SkCodec::kSuccess:
         case SkCodec::kIncompleteInput: