sched/tune: remove disabling sched_boost per cgroup

The current code support enabling/disabling sched boost per cgroup. This
is not used. Remove it. This also makes it unnecessary to track the
backup of enabled, remove it too.

Change-Id: I1af1bc9116a21e8921742a8aac6e9b8568e4de3c
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>