[S390] smp: remove cpu hotplug messages

Get rid of messages that indicate if a cpu went online or offline.
There is nothing special about this anymore and these messages might
flood the kernel log buffer which makes debugging harder since more
important messages might be overwritten.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>