cgroup: remove cgroup_attach_task_current_cg()

It's just a wrapper of cgroup_attach_task_all(), and it's no longer
used after commit 87d6a412
(vhost: fix attach to cgroups regression)

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>