How do I reverse clicking Trust this computer on non Apple PC

I know it's not a problem on my iPad but I accidentaly clicked on "Trust this computer" (Firefox on Ubuntu) and now can't log in to iCloud on this PC because, I presume, it expects verification it can't get from the PC. I've checked on my iPhone, iPad and windows PC and have no problem logging in on these devices so it's a problem of my own making on the one device. It's not life threatening but would be nice If someone knew how to reverse Trusting a non Apple computer.