cgroup: remove cgroup_pid_fry() and friends

cgroup_pid_fry() was added to mangle cgroup.procs pid listing order on
v2 to make it clear that the output is not sorted.  Now that v2 now
uses a separate "cgroup.procs" read method, this is no longer used.
Remove it.

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