MIAMI — A former professional boxer has been arrested after Miami police say he bought an AK-47 rifle and was preparing to shoot up a gym. Azea Agustama, 39, of North Miami, was jailed early Wednesday and charged with two felony counts of issuing written threats to commit a mass shooting, and one count of threatening to kill. Known in boxing as the “Haitian Hitman,” Agustama competed in the 2008 Olympics in China for Haiti, as a light heavyweight. He also notched 18 wins as a professional boxer, often appearing on cards at Casino Miami Jai-Alai and other venues in the mid-2010s. A judge on Wed…