The Dartmouth College basketball players who voted in March to unionize withdrew their petition on Tuesday, a move largely intended to preserve the favorable federal judgment that they had secured and that could have been in jeopardy once President-elect Donald J. Trump took office. The players were the first college athletes granted the right to unionize by a federal labor official and the ensuing 13-to-2 vote by the men’s team in favor of forming a union seemed like a pivotal moment in the mounting legal challenges to the college sports model. But Dartmouth refused to negotiate with the players and...