Remove the unnecessary call to `ob_end_clean()` directly after `ob_get_clean()` in `WP_Ajax_UnitTestCase::dieHandler()`. This fixes a large number of AJAX errors in debug mode.

See #25282.



git-svn-id: https://develop.svn.wordpress.org/trunk@25432 602fd350-edb4-49c9-b593-d223f7449a82