A teen was fatally stabbed during a clash on board a rush-hour train passing through the East Village on Thursday morning, cops said. The 19-year-old victim was knifed in the leg on the uptown L train at the First Avenue and East 14th Station around 8:10 a.m., authorities said.  The victim was taken to Bellevue Hospital, where he succumbed to his injuries, police said.  A person of interest was swiftly taken into custody with charges pending, cops said.  The attack stemmed from a skirmish