No, the edit wasn't necessary.
As other answers indicate, people can justify changing "who is one to believe?" into "which one to believe?", but I don't think the change is even a significant improvement, let alone a necessity.
Personally, I find "Which one to believe?" rather "clipped" and informal - it looks like a section heading in a magazine article, which doesn't sit well with the relatively formal style of the rest of the text.
The argument that OP is asking which reference source to believe, which therefore can't be "personified" using "who", seems trivial, bordering on pedantic. In any case it would have been a lesser edit to simply change the original "who" to "which". Deleting the word "is" makes an unwarranted change to the tone of the writing.
To summarise, I don't think the edit should have been made. It's irrelevant that some people prefer the revised version - the original certainly wasn't seriously defective, and in such matters I think common courtesy dictates that one should not make trivial changes to another's phrasing.
That last paragraph would be more appropriate if the question had been asked on meta, since it's about how the site should operate - but it's hard to separate "site protocol" from "strict grammar" on this particular issue, since the grammatical error (if indeed there is one) is so trivial.