Schering-Plough plans to cut 20% of its U.S. sales force by October 1 as part of a previously announced restructuring plan aimed at saving $1.5 billion by late 2012. The company said it will eliminate 1,000 sales positions from its U.S. Primary Care field force, which markets prescription drugs to physicians, and make organizational changes within the operation. According to the company, 4,000 positions will remain.     The restructuring program, announced in April, was designed in part to el...