cgroup: remove cftype->release()

Now that pidlist files don't use cftype->release(), it doesn't have
any user left.  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

diff --cc include/linux/cgroup.h
Simple merge

diff --cc kernel/cgroup.c
Simple merge