s390/cpum_sf: remove check on CPU being online

During sampling event initialization, a check is done if that
particular CPU the event is to be installed on is actually online.
This check is not necessary, as it is also performed in the
system call entry point. Therefore remove this check.
No functional change.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>