s390/docs: Remove sections that are not related to s390

Information how to use the GCC pre-processor, objdump, strace, top, etc.
are generic and not specific to the S390 architecture, so we do not need
this information in Debugging390.txt
Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>