sched, doc: Update sched-design-CFS.txt

Correct ->dequeue_tree() thinko into sched_class->dequeue_task
and drop all references to ->task_new() since it is obviously
gone.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <1300815978-16618-1-git-send-email-bp@amd64.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>