AFAIK, it's hardcoded in various localisation files etc - IE there is no in-game way to change the "flavour" of k_antioch from "principality" to something else. Since the "flavour" of Antioch is unique, people normally want to keep it and/or apply it to other titles... but you do you, I guess ;-p
(EDIT: For clarity, the principality of Antioch is king-tier. You're a king who calls himself a prince due to... humility? It's the same kind of idea as the king-tier ruler of Venice who calls himself "Doge" because "we're a republic and that's completely different from those nasty elective monarchies except not really".)
Probably the only thing you can do is create another kingdom title and use that as your primary title instead of Antioch. (If you are "King of Syria, Prince of Antioch" then you will be "King X of Syria".) Optionally, you could rename "Syria" to "Antioch" (requires customisation dlc) and de jure drift the entirety of k_antioch into k_syria; this gets you the title "King of Antioch", but you'll lose the unique map colour and coat of arms.
If you're willing to get into modding, it might be possible to change the unique localisation entries for Antioch back to the generic "King" etc. I think that changes to localisation don't affect the checksum, so this shouldn't block achievements.