Removed $GLOBALS['PIWIK_TRACKER_DEBUG_FORCE_SCHEDULED_TASKS']

Pull request for #6085

Removed `$GLOBALS['PIWIK_TRACKER_DEBUG_FORCE_SCHEDULED_TASKS']` and replaced it fully with `DEBUG_FORCE_SCHEDULED_TASKS`.

Explanation is in #6085
looks good - and i just check we do have a unit test checking the define
is not set
https://github.com/piwik/piwik/blob/master/tests/PHPUnit/Integration/ReleaseCheckListTest.php#L90-90