Welcome to the RingCentral Community

Unfortunately, there is no delete option to remove a developer from the list.

However, the dev support team can use the admin tool to remove any developers from the list.

You can make a list of the developers who you need to remove and submit the list in a dev support ticket and ask for help.

EDITED:

If you delete (first disable the extension) the unused user extension from your account using the admin portal, the developer portal will automatically remove and update the developer list!

I believe that this is your case where you allowed a 3rd party developer to access your RingCentral developer portal to create and manage apps.