remove U16 support, just support F16

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1889753002

Review URL: https://codereview.chromium.org/1889753002
diff --git a/src/core/SkCoreBlitters.h b/src/core/SkCoreBlitters.h
index ec9ee20..8fcfde6 100644
--- a/src/core/SkCoreBlitters.h
+++ b/src/core/SkCoreBlitters.h
@@ -185,13 +185,11 @@
     typedef SkShaderBlitter INHERITED;
 };
 
-SkBlitter* SkBlitter_ARGB32_Create(const SkPixmap& device, const SkPaint& paint,
-                                   SkShader::Context* shaderContext,
-                                   SkTBlitterAllocator* allocator);
+SkBlitter* SkBlitter_ARGB32_Create(const SkPixmap& device, const SkPaint&, SkShader::Context*,
+                                   SkTBlitterAllocator*);
 
-SkBlitter* SkBlitter_ARGB64_Create(const SkPixmap& device, const SkPaint& paint,
-                                   SkShader::Context* shaderContext,
-                                   SkTBlitterAllocator* allocator);
+SkBlitter* SkBlitter_F16_Create(const SkPixmap& device, const SkPaint&, SkShader::Context*,
+                                SkTBlitterAllocator*);
 
 ///////////////////////////////////////////////////////////////////////////////