s390/smp: keep the original lowcore for CPU 0

Now that CPU 0 is not hotpluggable, it is not necessary to support
freeing its stacks. Delete all the code that migrates it to new stacks
and a new lowcore.

Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>