RAQQA, Syria (Reuters) - U.S.-backed Syrian Democratic Forces (SDF) raised a militia flag inside Raqqa stadium on Tuesday, one of the last remaining areas that were held by Islamic State in its former capital, a Reuters witness said. The flag of the Kurdish YPG, the strongest of the militias in the SDF, was planted in the middle of the stadium, where fighting had ended, but which had not been fully cleared of landmines, militia fighters told the witness.  