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

This reverts commit 19f3e16b 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: Ieb0b69e6bbc195ab6b8fa2f955b5ded5a67f6e1c
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>