Although he has lost his job as chief executive of American Express Co., James D. Robinson III is trying to hang on as business' Mr. Trade in Washington. Robinson is chairman of the Advisory Committee for Trade Policy & Negotiations. The committee, representing the largest U.S. exporters, is an advisory panel recently set up by the White House. He also heads a business panel that advises the government on multilateral trade-liberalization talks. The White Hosue and U.S. Trade Representative Mickey Kantor will make the final decision on Robinson's fate.