s390/cpum_sf: Remove initialization of PMU event index

The git commit c719f560
"perf: Fix and clean up initialization of pmu::event_idx" removed
the PMU event index callback for all architectures but x86,
remove the initialization of the event index as well.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>