Bruce Welty never wanted to build robots; Amazon.com made him do it. Starting in the 1970s, Welty, 60, specialized in software for managing warehouses. In 2009, he shifted gears and founded Quiet Logistics, a company that used robots from Kiva Systems of North Reading to dramatically reduce warehouse operating costs. But three years later, retailing titan Amazon.com acquired Kiva, cutting off Welty’s robot supply. He saw only one hope — build his own robots.