move SkDither.h to private, remove unused shadeSpan16

src/effects/SkPerlinNoiseShader.cpp

Index: src/effects/SkPerlinNoiseShader.cpp
diff --git a/src/effects/SkPerlinNoiseShader.cpp b/src/effects/SkPerlinNoiseShader.cpp
index 51d84989a8cc72d543a1f126400974b319e54dc1..60e96017b2c903c696d084e3379cd4575f2fbd29 100644
--- a/src/effects/SkPerlinNoiseShader.cpp
+++ b/src/effects/SkPerlinNoiseShader.cpp
@@ -5,7 +5,6 @@
  * found in the LICENSE file.
  */
 
-#include "SkDither.h"
 #include "SkPerlinNoiseShader.h"
 #include "SkColorFilter.h"
 #include "SkReadBuffer.h"
@@ -467,19 +466,6 @@ void SkPerlinNoiseShader::PerlinNoiseShaderContext::shadeSpan(
     }
 }
 
-void SkPerlinNoiseShader::PerlinNoiseShaderContext::shadeSpan16(
-        int x, int y, uint16_t result[], int count) {
-    SkPoint point = SkPoint::Make(SkIntToScalar(x), SkIntToScalar(y));
-    StitchData stitchData;
-    DITHER_565_SCAN(y);
-    for (int i = 0; i < count; ++i) {
-        unsigned dither = DITHER_VALUE(x);
-        result[i] = SkDitherRGB32To565(shade(point, stitchData), dither);
-        DITHER_INC_X(x);
-        point.fX += SK_Scalar1;
-    }
-}
-
 /////////////////////////////////////////////////////////////////////
 
 #if SK_SUPPORT_GPU