[api] Remove duplicate forward declaration

Review-Url: https://codereview.chromium.org/2784653003
Cr-Commit-Position: refs/heads/master@{#44214}

include/v8.h

@@ -128,7 +128,6 @@ template<class V, class T> class PersistentValueVector;
 template<class T, class P> class WeakCallbackObject;
 class FunctionTemplate;
 class ObjectTemplate;
-class Data;
 template<typename T> class FunctionCallbackInfo;
 template<typename T> class PropertyCallbackInfo;
 class StackTrace;