I think we should keep both.
Both of the tags essentially serve the same function: they are warning flags for the solver, essentially saying
It is entirely possible that this puzzle is completely unsolvable to you.
This is critical information for the solver, because knowing when to give up on a difficult puzzle is never easy.
Out of the two tags, the knowledge tag is much less alarming: whatever knowledge is needed, it is not going to be of the obscure/useless/ill-defined variety, or if it is, it's at least going to have some scientific articles written about it.
The trivia tag is a much stronger warning; the question might require detailed knowledge of the evolutions of Pokey Men, plot details of all games in the Zelda series, or side characters in a TV show that has never even aired in your country. In other words, you might just be completely out of luck, no matter how hard you try.
Here's how I read the tags when solving:
knowledge "At some point, you are going to need Google, but you'll probably know what you need by then."
trivia "Feel free to google partial answers and guesses to get you on the right track. Also, be prepared to give up early if nothing seems to be making sense."
Since these are completely different meanings, with very distinct implications for the solver, I'd like to keep the tags separate.