Transform SELECT query to DELETE to remove extra taxonomy entries
If I unserstand the task correctly:


Delete "wrong" records


DELETE t1.*
FROM wp_term_relationships t1
WHERE t1.term_taxonomy_id IN (`none`,'low','medium','high')
  AND EXISTS ( SELECT NULL
               FROM wp_term_relationships t2
               WHERE t2.term_taxonomy_id IN ('none','low','medium','high')
                 AND t1.object_id = t2.object_id 
                 AND t1.term_order &gt; t2.term_order )




Forbid "wrong" records insertion:


ALTER TABLE wp_term_relationships 
ADD COLUMN term_taxonomy_check VARCHAR(8) 
           AS (CASE WHEN term_taxonomy_id IN (`none`,'low','medium','high')
                    THEN term_taxonomy_id END),
UNIQUE INDEX idx_term_taxonomy_check (object_id, term_taxonomy_check);