SO the much heralded visit of soap diva Joan Collins to Hongkong will not come to pass after all. The mercurial star of Dynasty , who was due here next month, has pulled out of the visit during which she was due to speak at a luncheon being held in aid of a children's charity. Joan's withdrawal will be a disappointment to Dancescope Asia director Daryl Ries who, despite some pitfalls, has worked hard to put together a schedule to meet the requirements of the British actress. ''The events involved will seek alternative dates with Ms Collins,'' Daryl said. Had she undertaken the trip, Joan would have spent her 60th birthday in Asia. Daryl added: ''For personal reasons she may wish to be at home for her birthday.'' Or it might be because STAR TV and the Sheraton Luncheon Club lost interest in sponsoring her visit.