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
I commented that out so that now to force the scheduled tasks we just have to uncomment it.