libcutils: remove schedgroup

After removing bg_non_interactive cgroup, there is only a ROOT cgroup.
This CL removes the no-op trying to set cgroup.
It also removes the attempt to open non-exist bg_non_interactive cgroup.

Bug: 36649832
Test: marlin boosts and cgroup ROOT task looks normal
Change-Id: Ibccd6db1499306174c940aec9219b20c1b86173d