I need to remove my old iphone from my account

If you are the original owner, contact Apple. Apple can remove the ID  if provided the required paperwork to include the original purchase receipt with the serial number on it. 

Request additional support for unlocking your device

Either take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened). Call the store before going to make sure they can help.

Genius Bar Reservation and Apple Support Options - Apple.    

or contact Apple ID Support. 

 Apple ID - Official Apple Support.    

Contact Apple for help with Apple ID account security - Apple Support.      (Link doesn’t always load)   

Get Support   https://getsupport.apple.com        Work through the screens to get a chat option. 

Apple Support schedule phone call        

https://twitter.com/AppleSupport