MAINTAINERS: remove Ursula Braun as s390 network maintainer

I am retiring soon. Thus this patch removes myself from the
MAINTAINERS file (s390 network).

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
[jwi: fix up the subject]
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>