s390/smp: do not preserve boot CPU lowcore on hotplug

Once the kernel is running the boot CPU lowcore becomes
freeable and does not differ from the secondary CPU ones
in any way. Make use of it and do not preserve the boot
CPU lowcore on unplugging. That allows returning unused
memory when the boot CPU is offline and makes the code
more clear.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>