Farm Convention Moved: Another agricultural group said it has pulled its convention out of San Francisco in retaliation for the city’s support of a table-grape boycott. The Produce Marketing Assn. is the sixth group to cancel a convention in San Francisco since the city’s Board of Supervisors voted in December to endorse a boycott of non-union table grapes sponsored by the United Farm Workers. The produce association’s executive board voted Feb. 12 to cancel its 1992 convention in San Francisco and consider dates in either Salt Lake City or Denver. The association had threatened to cancel the event if the board stood behind the boycott.