SINGAPORE — Russian defense companies have vanished from major airshows in Asia, which experts say reflects the country’s shrinking and increasingly concentrated arms-export industry. For the second consecutive year, Russian manufacturers were absent from the roster of attendees at the Singapore Airshow, the largest aerospace exhibition in Asia, which runs from Feb. 3-8. Moscow previously had a much larger presence at the event, even launching joint exhibition pavilions at the 2020 show for