OSLO (Reuters) - Norwegian Air's Bjoern Kjos stepped down on Thursday as chief executive of the airline he co-founded and turned into Europe's third-biggest budget carrier by passenger numbers.The airline has shaken up the long-haul market with cut-price transatlantic fares, but its rapid expansion has left it with hefty losses and high debts and it had to raise 3 billion crowns (£280 million) from shareholders earlier this year."I am way overdue," Kjos, 72, laughing, told a news conference,..