cgroup: remove stale item in cgroup-v1 document INDEX file.

Signed-off-by: Haosdent Huang <haosdent@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/cgroup-v1/00-INDEX

@@ -24,5 +24,3 @@ net_prio.txt
 	- Network priority cgroups details and usages.
 pids.txt
 	- Process number cgroups details and usages.
-unified-hierarchy.txt
-	- Description the new/next cgroup interface.