docs/zh_CN: remove zh-kernel.org in MAINTAINERS

This web site is on sale, and Li Yang said it won't be in service again.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Weiwei Jia <harryxiyou@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>