MAINTAINERS: Drop Li Yang as their email address stopped working

When sending a patch to (among others) Li Yang the nxp MTA replied that
the address doesn't exist and so the mail couldn't be delivered. The
error code was 550, so at least technically that's not a temporal issue.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>