IOWA CITY — After thrusting the University of Iowa’s College of Dentistry under the free speech microscope of Republican lawmakers with his October mass email condemning a White House executive order on diversity training, Dentistry Dean David Johnsen has announced he’ll step down after this semester. By leaving at the end of the college’s spring term in June 2021, Johnsen will depart one year earlier than expected — having announced in September plans to retire in summer 2022.