MAINTAINERS: Remove Martin Schwidefsky as s390 maintainer

https://lwn.net/ml/linux-kernel/20190521162350.GA17107@osiris/

ChangeLog:

2021-04-20  Andreas Krebbel  <krebbel@linux.ibm.com>

	* MAINTAINERS: Remove Martin Schwidefsky as s390 maintainer and
	add him to Past Maintainers.
	Update my email address.