Revert "cgroup: fix race between fork and cgroup.kill"

This reverts commit a00e6071 which is
commit b69bb476 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ib2ad473bfb9d038ead7593716eb498dbbef7beed
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>