The CEO of UnitedHealth’s insurance division was gunned down Wednesday morning outside the Hilton hotel in Midtown in what police say was a targeted attack.  Brian Thompson, 50, was at the hotel around 6:46 a.m., arriving early for a conference, when a masked man allegedly waiting for him fired at him repeatedly and fled eastbound off 6th Avenue, police sources told The Post.  Thompson was hit in the chest. He was rushed in critical condition to the Mt. Sinai Hospital...