LAS VEGAS — Vice President Mike Pence praised the heroic response by police and the resolve of the American people at a prayer service Saturday in Las Vegas before organizers released 58 white doves in memory of each victim killed in the deadliest mass shooting in modern U.S. history. At the same time, federal agents started hauling away piles of backpacks, baby strollers and lawn chairs left behind by fleeing concertgoers who scrambled to escape raining bullets from a gunman who was shooting from his high-rise hotel suite. “It was a tragedy of unimaginable proportions,” Pence said as he addressed nearly 300 people at Las Vegas City Hall.