Fix android sync

The android client will fail to get 

JSON response like {"modified":1371310063.91,"success":["7USxn4YvYwZI","dXu95gUYHkMb"],"failed":[]}

The main problem is "failed":[] instead of "failed":{}

But it will also fail when change all [] to {}

The solution is unset the unused variable to meet http://dxr.mozilla.org/mozilla-central/source/mobile/android/base/sync/repositories/Server11RepositorySession.java#l480


        JSONArray          success = body.getArray("success");
        if ((success != null) &&

        ExtendedJSONObject failed  = body.getObject("failed");
        if ((failed != null) &&