libprocessgroup: Remove schedtune support

Schedtune was an out of tree cgroup controller present in Android
kernels up to 4.19. It was replaced with uclamp and the cpu controller.
Now that 4.19 is EOL and unsupported, and schedtune is not present in
any supported Android kernels, remove the userspace support for
schedtune.

Bug: 362504801
Change-Id: If8baaf1a616984c5dbaac1bbd03e23ad001bc29c