Yama: remove needless CONFIG_SECURITY_YAMA_STACKED

Now that minor LSMs can cleanly stack with major LSMs, remove the unneeded
config for Yama to be made to explicitly stack. Just selecting the main
Yama CONFIG will allow it to work, regardless of the major LSM. Since
distros using Yama are already forcing it to stack, this is effectively
a no-op change.

Additionally add MAINTAINERS entry.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>