HONG KONG, Sept 30 (Reuters) - The value of announced acquisition deals for Asia-Pacific firms, excluding Japanese firms, dropped 13.4 percent to $790.3 billion in the first nine months of this year, according to preliminary data released by Thomson Reuters. Completed deals in the same period totalled $496.6 billion, posting a drop of 24.7 percent. Goldman Sachs was the top corporate adviser, working on $127 billion worth of transactions, followed by China International Capital Corp, and