s390/memory hotplug,sclp: get rid of per memory increment usecount

Given that the memory block size is always larger or equal to
the memory increment size there is no need for a usecount anymore.
Each memory increment belongs to exactly one memory block.
So get rid of the usecount.

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