LONDON (Reuters) - Sports Direct said its auditor Grant Thornton had quit, leaving the retailer struggling to find a replacement after it said last month that talks with other members of the "Big Four" accounting firms which audit most major companies had not been fruitful.Grant Thornton, the group's auditor since 2007, will not seek reappointment at Sport Direct's annual shareholders' meeting next month following a review of its client portfolio, the company said on Wednesday, without giving..