By Julien Ponthus and Elvira Pollina PARIS/MILAN (Reuters) -The collapse of a deal to form a French TV giant to challenge U.S. streaming services such as Netflix knocked shares in M6 Group and TF1 on Monday. France’s two biggest private broadcasters gave up their merger plan on Friday citing French antitrust requirements that rendered the deal unworkable. Like other local broadcasters in Europe, M6 and TF1 are struggling to stay competitive as global video platforms increase their dominance of the industry and a tie-up was seen as an answer to those challenges. “It is extremely disappointing, …