Walmart, the nation's biggest retailer, is pulling the plug on its health clinics, stepping back like other companies that have found that providing primary care to patients is a difficult and often unprofitable line of business. The Bentonville, Arkansas, company with 4,615 namesake U.S. stores said on Tuesday it is closing all 51 of its Walmart Health centers in five states — Arkansas, Florida, Georgia, Illinois and Texas — and shutting its telehealth unit.