BOSTON, June 5 (Reuters) - Two college friends of the Boston Marathon bomber were sentenced to prison on Friday for removing a backpack containing empty fireworks shells from Dzhokhar Tsarnaev's dorm room amid a massive manhunt three days after the deadly 2013 attack. Kazakh exchange student Azamat Tazhayakov was sentenced to 3-1/2 years for obstructing justice by taking, with his roommate, Tsarnaev's pack and dropping it in a dumpster by the off-campus apartment they shared. Robel Phillipos, of Cambridge, Massachusetts, was sentenced to three years in prison for lying to investigators about the incident, which occurred in the hours after...