Revert "s390/smp: enforce lowcore protection on CPU restart"

commit 95350375 upstream.

This reverts commit 6f5c672d.

This breaks normal crash dump when CPU0 is offline.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>