Heidelberg Cement said the start of a civil damages trial against it and five other German cement producers has been delayed until Dec 6.   The civil hearing in the Duesseldorf regional court, originally scheduled to begin on June 7, is being brought by several companies, which are seeking Euro 210m in damages for decades of pricing-fixing in the cement sector.   In 2003, the German Cartel Office saddled the cement companies with a Euro 661m fine for price-fixing, the largest penalty of its kind.   The cement producers are in the process of contesting the Cartel Office ruling.