Prince Norodom Sihanouk resigned as president of the U.N.-recognized Cambodian guerrilla coalition and canceled plans to attend a meeting of the alliance's three rebel groups later this month in Indonesia. Sihanouk announced his resignation in a statement released in Bangkok, Thailand, but gave no explanation. It was the second time in less than a year that the former Cambodian leader had quit as head of the coalition. At a meeting last week of the Assn. of Southeast Asian Nations, Sihanouk, who ruled Cambodia with various titles between 1941 to 1970, was touted as the key figure in diplomatic moves to end the nine-year conflict in Cambodia.