In a letter to the system's 13,200 employees, UMass Memorial Health Care chief executive John G. O'Brien blamed declines in health insurance payments and the number of patients it treats. He said the latest cuts will come on top of about the 150 positions that have been eliminated at UMass Memorial Medical Center in Worcester since February. The system also operates four community hospitals in central Massachusetts. O'Brien said UMass Memorial Health Care has completed the sale of its home health and hospice division to VNA Care Network & Hospice for an undisclosed price, shedding another 144 jobs from the system's payroll in the process.