By Aleksandar Vasovic, Ivana Sekularac and Edward McAllister BELGRADE, June 29 (Reuters) – When Aleksandar Vucic announced that he will step down as president of Serbia, cutting short his mandate and likely triggering early elections, few believed that he would disappear from politics. The populist leader, who has ruled as prime minister or president since 2014, has overseen a chapter of economic development but also growing rancour against perceived corruption and authoritarianism.