MAIDUGURI, Nigeria—The leader of Nigerian extremist group Boko Haram, Abubakar Shekau, has killed himself, according to a jihadi organization linked to the ISIS terrorist group. An audio recording purportedly from Abu Musab al-Barnawi, the leader of the ISIS West Africa Province, or ISWAP, said Shekau detonated explosives killing himself after a battle between the two…