No extra motivation needed when the hated Heat come to Boston. LeBron James’ 45-point masterpiece on their own floor that kept them from reaching the NBA Finals last year, Ray Allen swapping sides over the summer, and recent years of bad blood and great games would’ve had the Celtics ready Monday night. Now throw in the chance to end Miami’s 22-game winning streak. “If there’s any group that would be motivated in a circumstance like this, it’s that team in green,” Heat coach Erik Spoelstra said. “I don’t know if the streak will be a big factor, but they love …