Stop testing SkImageDecoder in DM/nanobench

dm/DMSrcSink.h

Index: dm/DMSrcSink.h
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 632744cd6209c9f449f0df13167fce63b37a2189..d02eeaf101e8c57604d058209d93ba0d20a7242a 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -180,18 +180,6 @@ private:
     uint32_t                                 fSampleSize;
 };
 
-class ImageSrc : public Src {
-public:
-    explicit ImageSrc(Path path);
-
-    Error draw(SkCanvas*) const override;
-    SkISize size() const override;
-    Name name() const override;
-    bool veto(SinkFlags) const override;
-private:
-    Path fPath;
-};
-
 class SKPSrc : public Src {
 public:
     explicit SKPSrc(Path path);