A birthday celebration for America’s 250th anniversary has become a political flashpoint after US President Donald Trump urged organizers to scrap a planned concert series and replace it with a rally led by him. "We should have a giant MAKE AMERICA GREAT AGAIN RALLY, for 250, instead of having overpriced singers, who nobody wants to hear, whose music is boring, and yet who do nothing but complain," Mr Trump posted on his Truth Social platform, adding: "Cancel it." The call came after several performers withdrew soon after…