Adam Kownacki knows his July 30 fight with Ali Eren Demirezen is a must-win. Adam Kownacki was a rising star for the PBC brand just two years ago, coming off of an exciting win over Chris Arreola in a heavyweight slugfest, and rumored to be right on track for a shot at the WBC heavyweight title. But two straight stoppage losses to Robert Helenius have derailed the Kownacki (20-2, 15 KO) hype train, and now the 33-year-old Polish-born Brooklynite is set to return on July 30, facing a tough Ali Eren Demirzen on Showtime. “This is the biggest fight of…