Documentation/s390: remove outdated debugging390 documentation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>

Documentation/s390/index.rst

@@ -7,7 +7,6 @@ s390 Architecture
 
     cds
     3270
-    debugging390
     driver-model
     monreader
     qeth