MAINTAINERS: s390/net: remove myself as maintainer

I won't have access to the relevant HW and docs much longer.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Link: https://lore.kernel.org/r/20211209153546.1152921-1-jwi@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>