Vernon Davis is a captain for the San Francisco 49ers, but the tight end's prolonged shouting match Wednesday with wide receiver Michael Crabtree crossed the line with coach Mike Singletary. As the Niners prepared for Thursday's exhibition finale, Singletary separated his receivers and eventually ushered both players off the field and into the locker room, according to the San Jose Mercury News. Crabtree has yet to play in this summer's exhibitions because of a neck injury, and the Mercury News suggests Davis was chewing out his second-year…