WASHINGTON — Of all the State Department employees who might have been vulnerable in the staff reductions that Secretary of State Rex W. Tillerson has initiated as he reshapes the department, the one person who seemed least likely to be a target was the chief of security, Bill A. Miller. Republicans pilloried Hillary Clinton for what they claimed was her inadequate attention to security as secretary of state in the months before the deadly 2012 attacks in Benghazi, Libya. Congress even passed legislation mandating that the department’s top security official have unrestricted access to the secretary of state. But in...