Clean Up

You should remove all records from the table tx_gridelements_backend_layout. Otherwise, users can still select the outdated grid elements.

Backup

Before you start: backup your tables:

pages
tx_gridelements_backend_layout

List configured grid elements

-- Get all folders
SELECT * 
FROM  pages
WHERE doktype = 254 
AND uid IN (
	SELECT `pid`
	FROM `tx_gridelements_backend_layout`
	GROUP BY `pid`
	ORDER BY `pid`
);

-- Get all configuration records
SELECT `uid`, `pid`, `title`
FROM `tx_gridelements_backend_layout`;

Remove configured grid elements

-- First: delete pages
DELETE FROM  pages
WHERE doktype = 254
AND uid IN (
	SELECT `pid`
	FROM `tx_gridelements_backend_layout`
	GROUP BY `pid`
	ORDER BY `pid`
);

-- Second: delete records
DELETE FROM `tx_gridelements_backend_layout`;