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

This reverts commit 28e51dd4 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: I679df65c191344d69cdde76cba10b66ba2bf8e1e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>