Here the printer’s manuscript lacks the word away; the 1830 edition has it. In accord with the reading in 𝓟, the 1908 RLDS edition adopted the reading without the away. Elsewhere in the text there are 38 other occurrences referring to a number of years having elapsed—and in each case, the phraseology is “passed away”, never simply “passed”. Most likely, 𝓞 read “passed away” and scribe 2 of 𝓟 accidentally dropped the word away when he copied from 𝓞 into 𝓟.
Summary: Maintain the use of away in Mormon 8:6: “four hundred years have passed away”, the 1830 reading; usage elsewhere in the text consistently supports the occurrence of away when referring to the passage of years.