How to remove two factor authentication

You can try contacting Apple ID Support to see if they can/will assist you..

Apple ID Support

Apple ID Support (2) 

Apple Support by e-mail or chat    Work through the screens to get a chat option.  You may need to pick a problem that is slightly different than yours to get a chat option.  

Apple Support using Twitter

Or fill out and submit this form.          The form is for iTunes, but it will get passed to the ID Support section.  

From the article below. If it has been on longer than 2 weeks, you can't turn it off. 

Can I turn off two-factor authentication after I’ve turned it on?

If you already use two-factor authentication, you can no longer turn it off. Certain features in the latest versions of iOS and macOS require this extra level of security, which is designed to protect your information. If you recently updated your account, however, you can unenroll for a period of two weeks. Just open your enrollment confirmation email and click the link to return to your previous security settings. Keep in mind, this makes your account less secure and means that you can't use features that require a higher level of security.

Apple ID -Two-factor authentication

You cannot remove 2 factor authentication.

It is Apple policy that it cannot be turned off or removed.