Remove stray ImpalaServer::CreateDataSink() declaration.

The definition doesn't exist. The real one is DataSink::CreateDataSink.

Change-Id: I1f2174ff66e2196b363658eb36ed0eb78f8462a5
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4310
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins