The Wikipedia article reads:
After a reign of thirty-seven years, Romulus is said to have disappeared in a whirlwind during a sudden and violent storm, as he was reviewing his troops on the Campus Martius. Livy says that Romulus was either murdered by the senators, torn apart out of jealousy, or was raised to heaven by Mars, god of war. Livy believes the last theory regarding the legendary king's death, as it allows the Romans to believe that the gods are on their side, a reason for them to continue expansion under Romulus' name.
So it would appear that Romulus vanished, and his body was not found. He was officially declared to have been assumed into heaven (apotheosis ). This was intended to rebut rumors that he had been murdered by patricians.
Thus "taken off" here would seem to mean "abducted, and (probably) killed afterwards".