BRUSSELS—Europe’s tech chief Margrethe Vestager on Friday warned iPhone maker Apple against using privacy and security concerns to fend off competition on its App Store, reasons CEO Tim Cook gave for not allowing users to install software from outside the Store. Vestager, who is also the European Commission’s executive vice president, last year proposed rules called the Digital Markets Act (DMA) that would force Apple to open up its lucrative App Store so that users can download apps from the internet or third-party app stores in a practice known as side-loading. Cook, speaking at an event last month, said the proposal would destroy the security and privacy of iPhones. Vestager said she shares Cook’s security concerns. “I think privacy and security is of paramount importance to everyone,” Vestager told Reuters in an interview. “The important thing here is, of course, that it’s not a shield against competition, because I think …