MAINTAINERS: update S390 NETWORK DRIVERS maintainer

remove Frank Blaschka as S390 NETWORK DRIVERS maintainer

Acked-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>