As part of a restructuring plan it announced four months ago, Merck said it will eliminate 50 jobs at a site in Springfield that it gained through a merger with Schering-Plough. In a notice filed with the state Department of Labor earlier this week, Merck said the job cuts would go into effect on Dec. 20. The notice did not describe the type of jobs Merck is cutting at the Lawrence Road office site, and Ron Rogers, a spokesman for the drug maker, said the company would not provide specific details about the functions being eliminated.