Remove obsolete kIndex_8_SkColorType reference

Bug: skia:6828

No public API change. Just removing an obsolete comment.
TBR=reed@google.com

Change-Id: I6c729c62bf7d94fc6acb677d68e7091f80d73b3e
Reviewed-on: https://skia-review.googlesource.com/150781
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>

include/core/SkImageGenerator.h

@@ -78,8 +78,6 @@ public:
      *         to scale. If the generator cannot perform this scale,
      *         it will return false.
      *
-     *         kIndex_8_SkColorType is not supported.
-     *
      *  @return true on success.
      */
     struct Options {};