Remove orphaned SkImageDecoder::Target.

This was used by an experimental method on SkImageDecoder, which no
longer exists.

Review URL: https://codereview.chromium.org/760923003
diff --git a/include/core/SkImageDecoder.h b/include/core/SkImageDecoder.h
index b384708..d720f4e 100644
--- a/include/core/SkImageDecoder.h
+++ b/include/core/SkImageDecoder.h
@@ -313,21 +313,6 @@
         return DecodeMemory(buffer, size, bitmap, kUnknown_SkColorType, kDecodePixels_Mode, NULL);
     }
 
-    /**
-     *  Struct containing information about a pixel destination.
-     */
-    struct Target {
-        /**
-         *  Pre-allocated memory.
-         */
-        void*  fAddr;
-
-        /**
-         *  Rowbytes of the allocated memory.
-         */
-        size_t fRowBytes;
-    };
-
     /** Decode the image stored in the specified SkStreamRewindable, and store the result
         in bitmap. Return true for success or false on failure.