LONG BEACH, Calif. — At 45, the legendary Renzo Gracie has no real reason left to fight. His reputation as one of mixed martial arts’ founding fathers is secure. He has well-earned status as jiu-jitsu’s ever-smiling ambassador, and his various business concerns, his famed Manhattan gym chief among them, keep him busy and active. But Gracie is, at heart, still a fighter. And in between posing for pictures with fans out on the floor of the Long Beach Convention Center at the World Jiu-Jitsu Expo…