CAIRO — In an eerie coincidence, the EgyptAir jetliner that plunged into the Mediterranean on Thursday was once the target of political vandals who wrote in Arabic on its underside, “We will bring this plane down.” Three EgyptAir security officials said the threatening graffiti, which appeared about two years ago, had been the work of aviation workers at Cairo Airport. Playing on the phonetic similarity between the last two letters in the plane’s registration, SU-GCC, and the surname of Egypt’s president, Abdel Fattah el-Sisi, some workers also wrote “traitor” and “murderer.”