By Grant McCool NEW YORK, June 10 (Reuters) - Former Goldman Sachs Group Inc board member Rajat Gupta, on trial on insider trading charges, has decided not to testify in his own defense, two days after his lawyer said it was "highly likely" he would take the stand. "After substantial reflection and consideration, we have determined that Mr Gupta will not be a witness on his own behalf in the defense case," Gupta's main lawyer, Gary Naftalis, wrote in a letter to U.S.