You're thinking about the Rich Text Editor. That's the big ugly box you typed that post in with all the buttons around it, and you toggle it off in your profile. It's possible you'll need to log out and then in again for the change to take effect.
That will mean you just need to use HTML tags for italics and bold in your stories. 95% of the time that's all the formatting anybody bothers with anyway, though if you know more advanced stuff about HTML there's a lot more flexibility there.
And you might as well grab the site extension from
Brad's profile while you're at it btw, now with the RTE box gone you can give dark mode a try. (It's toggled on in your profile.)