s390: remove generic email address from maintainers file

The public mailing lists and personal email addresses are sufficient.
No need for an extra generic email address.

Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Ursula Braun <ursula.braun@de.ibm.com>
Cc: Ingo Tuchscherer <ingo.tuchscherer@de.ibm.com>
Cc: Steffen Maier <maier@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>