Revert "Kconfig: set the default of CGROUP_TIMER_SLACK to y."

This reverts commit 7cfdcb078267a91d8c5b8fa067e2d4f7caa57fc4.
Revert "kernel: Introduce timer slack controller."

This reverts commit 61afd16bf9293e5eee8cb28dc423748286d66d0f.