Tables used :
APEX_XXXXXXX.WWV_FLOW_REPORT_LAYOUTS
APEX_XXXXXXX.WWV_FLOW_SHARED_QUERIES
use the following query to delete unused Layouts.
make sure you do have backup before .
DELETE FROM APEX_220100.WWV_FLOW_REPORT_LAYOUTS ll
WHERE NOT EXISTS
(SELECT 1
FROM APEX_220100.WWV_FLOW_SHARED_QUERIES qq
WHERE qq.REPORT_LAYOUT_ID = ll.id);