SANTA CLARA, Calif. — On the day the San Francisco 49ers were officially eliminated from playoff contention, they delivered a performance that summed up their disappointing season. The offense failed to reach 20 points for the seventh time this season, a missed field goal hampered a comeback attempt and the defense got gashed on the ground in a 29-17 loss to the Miami Dolphins on Sunday. “It’s just been not clean football across the board,” quarterback Brock Purdy said.