The acting head of U.S. Immigration and Customs Enforcement, whose appointment to the post raised eyebrows because of his scant law enforcement experience, told agency employees he will resign Friday, FoxNews.com has learned. John Sandweg, a former defense attorney who knew former Department of Homeland Security Secretary Janet Napolitano from their days as political allies in Arizona and came to Washington with her, gave his notice just six months after taking the reins in August. In a letter Friday to coworkers, Sandweg said he will return to work in the private sector but gave no explanation for his departure. “Over...