Rachel L. Brand, the No. 3 official at the Justice Department, plans to step down after nine months on the job as the country’s top law enforcement agency has been under attack by President Trump, according to two people briefed on her decision. Ms. Brand’s assistant, Currie Gunn, has also left the department. Ms. Gunn could not be reached for comment. In her current job, she reports directly to Mr. Rosenstein and Jeff Sessions, the attorney general, who has recused himself from the Russia investigation.