RBS Securities – which is 64 percent owned by the government of the United Kingdom – is shaking up its mortgage trading operation in the U.S., cutting staff and taking a close look at its future in an extremely tough American mortgage market. Officials at the bank’s MBS headquarters in Stamford, CT, did not return telephone calls about the matter, but several lenders and Wall Street executives confirmed that cutbacks have been made at the company over the past week or so. Frank Skibo, a managing director for RBS in Connecticut, and Ara Balabanian, a director in the group, also could not be reached...