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>

Documentation/scheduler/00-INDEX

@@ -4,8 +4,6 @@ sched-arch.txt
 	- CPU Scheduler implementation hints for architecture specific code.
 sched-coding.txt
 	- reference for various scheduler-related methods in the O(1) scheduler.
-sched-design.txt
-	- goals, design and implementation of the Linux O(1) scheduler.
 sched-design-CFS.txt
 	- goals, design and implementation of the Complete Fair Scheduler.
 sched-domains.txt