MAINTAINERS: Drop the line with Xiang Zheng

MAINTAINERS: Drop the line with Xiang Zheng

When sending a mail with CC: to zhengxiang9@huawei.com I got a mail
back saying:

 Your message couldn't be delivered

 The message you sent to zhengxiang9@huawei.com couldn't be delivered due to:
 Recipient email address is possibly incorrect.

 Further information

 5.1.1 <zhengxiang9@huawei.com>: Recipient address rejected: Failed recipient
 validation check.: host 127.0.0.1[127.0.0.1] said: 554 5.7.1 recipient verify
 from ldap failed (in reply to RCPT TO command)

Looks like this address does not exist anymore, thus drop it from the
MAINTAINERS file.

Message-Id: <20210401065343.3197791-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>