Talks between Los Angeles County and the largest of the county's 21 employee unions continued with no settlement in sight. Leaders of Service Employees International Union, Local 660, who had set a midnight Monday deadline for an agreement, said they would not call a walkout of their members. Instead, noontime demonstrations are expected today at area welfare offices, several county hospitals and a number of public health centers. Management officials said that they presented an offer and that the union, which represents about 40,000 employees, was studying it. Local 660 is the only union that has not reached tentative agreement with the county.