Remove some references to defunct kIndex_8

Bug: skia:6828
Bug: skia:3338
Bug: skia:3339

The comment for five_reference_pixels seems to suggest removing
the default case, but we already have new SkColorTypes, leaving
the default seems like the right approach for now.

Remove premultiply_if_necessary. We can now draw unpremultiplied
bitmaps, so it's never necessary.

Remove a TODO related to kIndex8

Change-Id: Iafdab22bb6453e4b5bba7278b12d27788cc7d037
Reviewed-on: https://skia-review.googlesource.com/151362
Commit-Queue: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/tests/ReadPixelsTest.cpp b/tests/ReadPixelsTest.cpp
index 4079a8c..29e50ff 100644
--- a/tests/ReadPixelsTest.cpp
+++ b/tests/ReadPixelsTest.cpp
@@ -600,7 +600,7 @@
         case kRGBA_F16_SkColorType:
             return f16;
         default:
-            return nullptr; // remove me when kIndex_8 is removed from the enum
+            return nullptr;
     }
 
     SkASSERT(false);