Fix TestSQLBackend.testRecursiveRemoval on MySQL

org.nuxeo.ecm.core.storage.StorageException: Could not delete: hierarchy
	at org.nuxeo.ecm.core.storage.sql.jdbc.JDBCRowMapper.deleteRowsDirect(JDBCRowMapper.java:909)
	at org.nuxeo.ecm.core.storage.sql.jdbc.JDBCRowMapper.remove(JDBCRowMapper.java:1326)
	at org.nuxeo.ecm.core.storage.sql.SoftRefCachingRowMapper.remove(SoftRefCachingRowMapper.java:504)
	at org.nuxeo.ecm.core.storage.sql.PersistenceContext.removeNode(PersistenceContext.java:916)
	at org.nuxeo.ecm.core.storage.sql.SessionImpl.removeNode(SessionImpl.java:1089)
	at org.nuxeo.ecm.core.storage.sql.TestSQLBackend.testRecursiveRemoval(TestSQLBackend.java:278)