s390/zcore: remove /sys/kernel/debug/zcore/mem

New versions of the SCSI dumper use the /dev/vmcore interface instead
of zcore mem. Remove the outdated interface.

Acked-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>