Fix using XML mapping driver with XSD validation disabled is deprecated

[raziel057]

Fix using XML mapping driver with XSD validation disabled is deprecated and will not be supported in Doctrine ORM 3.0.

12x: Using XML mapping driver with XSD validation disabled is deprecated and will not be supported in Doctrine ORM 3.0. (XmlDriver.php:60 called by SimplifiedXmlDriver.php:23, https://github.com/doctrine/orm/pull/6728, package doctrine/orm)
    10x in MeetingControllerTest::testCompleteScenario from PTC\AdminBundle\Tests\Controller
    1x in MeetingControllerTest::testCalendarView from PTC\AdminBundle\Tests\Controller
    1x in MeetingControllerTest::testDeleteWarningAction from PTC\AdminBundle\Tests\Controller

See doctrine/DoctrineBundle#1647

Please note that validate_xml_mapping needs to be set on true to expose the XSD validation in the DoctrineOrmMappingsPass

https://www.doctrine-project.org/projects/doctrine-bundle/en/latest/configuration.html
doctrine/DoctrineBundle#1679