src: remove unnecessary forward declarations

PR-URL: #633
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>

Author: Gabriel Schulhof

napi.h

@@ -133,11 +133,9 @@ namespace Napi {
   class Array;
   class ArrayBuffer;
   class Function;
-  template <typename T> class Buffer;
   class Error;
   class PropertyDescriptor;
   class CallbackInfo;
-  template <typename T> class Reference;
   class TypedArray;
   template <typename T> class TypedArrayOf;