By plugin hooked into applications/nexus/sources/Support/Request.php
/* !Forms */
/**
* Get elements for add/edit form
*
* @param \IPS\Content\Item|NULL $item The current item if editing or NULL if creating
* @param \IPS\Node\Model|NULL $container Container (e.g. forum), if appropriate
* @return array
*/
public static function formElements( $item=NULL, \IPS\Node\Model $container=NULL )
{
I affraid you can't remove it. It would be bad idea anyway.
Custom Fields are available in the Support Settings: