selinux: update comment on selinux_hooks[]

After commit f22f9aaf ("selinux: remove the runtime disable
functionality"), the comment on selinux_hooks[] is out-of-date,
remove the last paragraph about runtime disable functionality.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>