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
Yes I replaced that unit test with that:

```php
$this->assertFalse(defined('DEBUG_FORCE_SCHEDULED_TASKS'));
```