http2: adjusted to newer nghttp2_session_callbacks struct

the number of elements in the 'nghttp2_session_callbacks' structure is
now reduced by 2 in version 0.3.0 (I'm not sure when the change
happened, but checking for ver 0.3.0 work for me).

lib/http2.c

@@ -141,8 +141,6 @@ static const nghttp2_session_callbacks callbacks = {
   NULL,
   NULL,
   NULL,
-  NULL,
-  NULL,
   NULL
 };