Remove XFAIL

This was fixed in PHP 7 according to https://bugs.php.net/bug.php?id=61390 (which the linked commit points to)

Author: Girgias

ext/dba/tests/dba_db4_018.phpt

@@ -54,8 +54,6 @@ echo dba_fetch("key2", $db_file1), "\n";
 <?php
     require(__DIR__ .'/clean.inc');
 ?>
---XFAIL--
-Test 6 crashes with dba pointer of NULL, bug http://bugs.php.net/bug.php?id=51278
 --EXPECTF--
 database handler: db4
 Test 1