L’AQUILA, Italy — Matteo Messina Denaro, a convicted mastermind of some of the Sicilian Mafia’s most heinous slayings, died on Monday in a hospital prison ward, several months after being captured as Italy’s No. Matteo Messina Denaro, a convicted mastermind of some of the Sicilian Mafia’s most heinous slayings, died on Monday in a hospital prison ward, several months after …