Documentation/s390: remove outdated debugging390 documentation

This file would need a lot of work to make sense again. Thomas Huth
started working on that four years ago, but that wasn't finished.
Therefore remove this.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>