By Veronika Bondarenko Retail giant Walmart is revamping an often overlooked product line that drives big sales. Walmart bases much of its appeal to shoppers on low prices for many necessities. In the search for growth, the retail giant has gone after the grocery and club membership businesses in recent years. But sometimes just renewing an old business line can help the cause as well. In the 23 years that it’s been around, Walmart (WMT) – Get Walmart Inc. Report’s first sleepwear and intimates line has become a major source of sales. The Secret Treasures line brought in over $1 billion in 202…