cgroup: Remove stale cpu.rt.max reference from documentation

cpu.rt.max was a proposed interface that never landed in mainline. Remove the
reference from cgroup-v2 documentation.

Signed-off-by: Tejun Heo <[email protected]>
Reported-by: Orestis Floros <[email protected]>

Author: htejun

Documentation/admin-guide/cgroup-v2.rst

@@ -737,9 +737,6 @@ combinations are invalid and should be rejected.  Also, if the
 resource is mandatory for execution of processes, process migrations
 may be rejected.
 
-"cpu.rt.max" hard-allocates realtime slices and is an example of this
-type.
-
 
 Interface Files
 ===============