SIOUX FALLS, S.D. — The head of one of the largest regional health systems in the Midwest was replaced Tuesday, less than a week after telling employees that he had recovered from the novel coronavirus and was not wearing a mask around the office.Sanford Health said in a release that it has “mutually agreed to part ways” with longtime CEO Kelby Krabbenhoft, who took over in 1996 and helped expand the organization from a community hospital into what is billed as the largest rural nonprofit health system in the country.