A teenager was shot to death in a brazen midday attack Wednesday near Northeastern University in Roxbury, police said. Officer Katherine Shea, a Boston police spokeswoman, said a 19-year-old man was shot at 1053 Tremont St., which is near the Northeastern campus and the Ruggles MBTA Station, shortly before 1:30 p.m. The victim, whom police did not identify, was taken to Boston Medical Center, where he was pronounced dead, Shea said. No suspects were in custody Wednesday night, Shea said. In an e-mail to students, Northeastern said that the shooting was unrelated to the school and that there is no threat to students.