These commands will delete records from the tables field_data_comment_body and field_data_revision_comment_body, so be advised to make a backup first!

DELETE t1.*
FROM field_data_comment_body t1
    LEFT JOIN comment t2 ON t1.entity_id = t2.cid
WHERE t2.cid IS NULL

DELETE t1.*
FROM field_revision_comment_body t1
    LEFT JOIN comment t2 ON t1.entity_id = t2.cid
WHERE t2.cid IS NULL

OPTIMIZE TABLE `field_data_comment_body` ,
`field_revision_comment_body`