STOCKHOLM, Jan 17 (Reuters) - Swedish wrestler Mikael Ljungberg, who won the gold medal at the 2000 Sydney Olympics in the 97 kg event, has retired  because of a shoulder injury.     "I have been into rehabilitation but haven't recovered completely. Sometimes I'm in real pain," the 31-year old told Swedish television. "Now is the time to quit."     The Swede also won the World Championships in 1999, 1995 and 1993.