-- Deputy Trade Representative Rufus H. Yerxa--the chief U.S. negotiator for the new General Agreement on Tariffs & Trade--plans to leave his post early next year. Yerxa, 43, who in 1981 became trade counsel to former House Ways & Means Committee Chairman Dan Rostenkowski (D-Ill.), will return to his roots in Seattle--a gateway to Asia--to practice trade law. Meanwhile, despite rumors to the contrary, the other deputy USTR, Charlene Barshefsky, insists she is not leaving the government. But friends say that she has considered quitting because of frustration over the Administration's inability to reach a far-reaching trade agreement with Japan. Talks continue in autos and a few other sectors.