sched: remove sched-design.txt from 00-INDEX

Impact: remove stale documentation reference

sched-design.txt has been removed.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>