The head of Greece’s Radical Left Coalition, whose strong gains in last month’s elections deepened concerns over his country’s future in the European single currency, vowed Friday to cancel Greece’s international bailout agreement if he wins an upcoming repeat ballot. Alexis Tsipras, whose Syriza party came second in the inconclusive May 6 polls, said there was no way to partially implement the terms of the bailout that the heavily indebted country has come to rely on to pay its way.