Reland "Remove support for decoding to kIndex_8"

Original change's description:
> > 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>
> 
> TBR=djsollen@google.com,scroggo@google.com
> 
> Change-Id: I1bc669441f250690884e75a9a61427fdf75c6907
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: skia:6828
> Reviewed-on: https://skia-review.googlesource.com/22120
> Reviewed-by: Leon Scroggins <scroggo@google.com>
> Commit-Queue: Leon Scroggins <scroggo@google.com>

TBR=djsollen@google.com,scroggo@google.com

Bug: skia:6828
Change-Id: I36ff5a11c529d29e8adc95f43b8edc6fd1dbf5b8
Reviewed-on: https://skia-review.googlesource.com/22320
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
diff --git a/src/codec/SkBmpCodec.cpp b/src/codec/SkBmpCodec.cpp
index c21a863..3142f1e 100644
--- a/src/codec/SkBmpCodec.cpp
+++ b/src/codec/SkBmpCodec.cpp
@@ -622,19 +622,19 @@
 }
 
 SkCodec::Result SkBmpCodec::prepareToDecode(const SkImageInfo& dstInfo,
-        const SkCodec::Options& options, SkPMColor inputColorPtr[], int* inputColorCount) {
+        const SkCodec::Options& options) {
     if (!conversion_possible(dstInfo, this->getInfo()) ||
         !this->initializeColorXform(dstInfo, options.fPremulBehavior))
     {
         return kInvalidConversion;
     }
 
-    return this->onPrepareToDecode(dstInfo, options, inputColorPtr, inputColorCount);
+    return this->onPrepareToDecode(dstInfo, options);
 }
 
 SkCodec::Result SkBmpCodec::onStartScanlineDecode(const SkImageInfo& dstInfo,
-        const SkCodec::Options& options, SkPMColor inputColorPtr[], int* inputColorCount) {
-    return prepareToDecode(dstInfo, options, inputColorPtr, inputColorCount);
+        const SkCodec::Options& options) {
+    return prepareToDecode(dstInfo, options);
 }
 
 int SkBmpCodec::onGetScanlines(void* dst, int count, size_t rowBytes) {