nghttpx: Don't consume response data in downstream on_stream_close_callback

Even after on_stream_close_callback, Http2DownstreamConnection is
still alive and upstream keeps sending response to the client.  The
consumed bytes are processed normally (data_source_read_callback) and
also we have a code to consume all allocated bytes for
Http2DownstreamConnection object when it is deleted.  This means that
we don't need to and should not consume response data in downstream
on_stream_close_callback.  If we do, we may get assertion error in
Http2DownstreamConnection::resume_read().