MAINTAINERS: remove non-exsiting email address of Baoyou

We keep getting bounces from Baoyou Xie <baoyou.xie@linaro.org>.  We
do not know Baoyou's new email address, nor his interest to stay as
the co-maintainer.  Let's remove the non-exsiting email first, and we
can add his new email back if we heard back from him later.

Cc: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>