General Mills Inc. last year wrote off more than 25 percent of the intangible value of its Progresso line of products — Campbell’s most prominent competitor in the soup aisle. In dollar terms, that was a $132 million haircut. With that move, General Mills joined the parade of companies writing down value of old staple brands. The latest of Kraft Heinz’s well-publicized write-downs were on Miracle Whip and Maxwell House, for example.