HISTORY, it may seem, is a load of junk. That was what we thought when we heard the lively Central hostelry, Mad Dogs, had received a six-inch World War II shell from the Bomb Disposal Unit of the Hong Kong Police. Pub manager Irene Weaving said: ''We were pleased to accept this bit of Hong Kong history since it is in keeping with our tradition of displaying unusual artefacts.'' The shell was recovered by an American dredger during reclamation work connected with the Chek Lap Kok airport project. But with recent stories going around that the Far East Exchange Building, where Mad Dogs is located, is about to be subject to marauding bulldozers, here's hoping the latest addition to the Mad Dogs archives does not do the job for them.