s390x: smp: Remove unneeded cpu loops

Now that we have a loop which is executed after we return from the
main function of a secondary cpu, we can remove the surplus loops.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200429143518.1360468-7-frankja@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>