Delete dead subset test code from dm

dm/DMSrcSink.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef DMSrcSink_DEFINED
 #define DMSrcSink_DEFINED
 
@@ skipping 161 matching lines @@
 private:
     Path                                     fPath;
     SkBitmapRegionDecoder::Strategy          fStrategy;
     Mode                                     fMode;
     CodecSrc::DstColorType                   fDstColorType;
     uint32_t                                 fSampleSize;
 };
 
 class ImageSrc : public Src {
 public:
-    // divisor == 0 means decode the whole image
-    // divisor > 0 means decode in subsets, dividing into a divisor x divisor grid.
-    explicit ImageSrc(Path path, int divisor = 0);
+    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;
-    const int  fDivisor;
 };
 
 class SKPSrc : public Src {
 public:
     explicit SKPSrc(Path path);
 
     Error draw(SkCanvas*) const override;
     SkISize size() const override;
     Name name() const override;
 private:
@@ skipping 159 matching lines @@
 
 class ViaTwice : public Via {
 public:
     explicit ViaTwice(Sink* sink) : Via(sink) {}
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
 };
 
 }  // namespace DM
 
 #endif//DMSrcSink_DEFINED