kselftest/cgroup: Simplify test_cpuset_prs.sh by removing "S+" command

The "S+" command is used in the test matrix to enable the cpuset
controller. However this can be done automatically and we never use the
"S-" command to disable cpuset controller. Simplify the test matrix and
reduce clutter by removing the command and doing that automatically.
There is no functional change to the test cases.

Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: Waiman-Long