The Venn diagram of top executives and amateur mixed martial artists doesn’t feature much crossover, but Professional Fighters League can count its CEO in that middle overlapping area. “I wasn’t that great as a fighter,” John Martin, who took the helm of the MMA promotion last July, conceded Wednesday to The Post with a chuckle after rattling off his past martial arts training in the disciplines of karate, jiu-jitsu, Muay Thai and judo, “but I loved it. And I did all this while I was working