If you don't have any assignments or grades in the class (or it's okay for them to be deleted), you can go to https://www.khanacademy.org/profile/me/teachers and remove any teachers (this will also remove you from their classes)
https://support.khanacademy.org/hc/en-us/articles/47239348217229-How-can-I-go-back-to-the-classic-Khan-Academy-experience-as-an-independent-learner

~ Janice, KA user