Samson Khachatryan, a boxer who won the European Championship twice and was crowned Soviet Union champ six times, was kidnapped by masked men yesterday at the Dynamo sports stadium.   His wife, Nazik, knew nothing of the kidnapping until she heard the news from friends in Vanadzor, Samson’s home town. All she could tell Hetq was that the kidnapping could have something to do with her husband’s political activism; he’s a member of HAK (Armenian National Congress) opposition alliance. She did mention that last evening some 4-5 individuals in civilian clothes came to her house in Yerevan and began to search the place.