New user is assigned 2 roles: customer and superadmin

To restore super admin to just the admin username:

<?php update_site_option( 'site_admins', array('admin') ); ?>

Place a PHP file with this line in your /wp-content/mu-plugins/ directory and access any site URL. Once done you can delete the file.

Alternative you can edit the site_admins key directly in the wp_sitemeta table (but the first is easier as it will be serialized data.)

You probably want to check your default_role options also, as well as checking for other user role and capabilities in case they have been changed too.