Philippine authorities have arrested six suspected militants from the Islamic State-linked Abu Sayyaf group, in a sign of how the terrorist network continues to trouble Southeast Asia even as its influence wanes in the Middle East.Police on Tuesday said they had raided Islamist hideouts days after the killing of Dutch wildlife photographer Ewold Horn on May 31, when insurgents holding the 59-year-old hostage clashed with government security forces in the restive southern Philippines.Horn was…