WASHINGTON — The top U.S. diplomat for Europe is resigning after only 16 months on the job in a blow to efforts to maintain trans-Atlantic unity amid questions about the Trump administration’s commitment to historic alliances. The State Department said Tuesday that Assistant Secretary of State for Europe and Eurasian Affairs Wess Mitchell would leave his post in mid-February. “Wess has done an outstanding job as assistant secretary,” Secretary of State Mike Pompeo said.