Inspired by ZBX-4227: in trunk, it is not possible to delete an active maintenance due to foreign key constraints:

Error in query [DELETE FROM maintenances WHERE (maintenanceid IN ('2')) ] [Cannot delete or update a parent row: a foreign key constraint fails (`issue_dev_502`.`hosts`, CONSTRAINT `c_hosts_2` FOREIGN KEY (`maintenanceid`) REFERENCES `maintenances` (`maintenanceid`))]
SQL statement execution has failed "DELETE FROM maintenances WHERE (maintenanceid IN ('2')) "

Before deleting a maintenance, the GUI has to reset fields related to maintenance in the hosts table.