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>

arch/s390/kernel/perf_cpum_sf.c

@@ -1383,7 +1383,6 @@ static int cpumsf_pmu_add(struct perf_event *event, int flags)
 		cpuhw->lsctl.ed = 1;
 
 	/* Set in_use flag and store event */
-	event->hw.idx = 0;	  /* only one sampling event per CPU supported */
 	cpuhw->event = event;
 	cpuhw->flags |= PMU_F_IN_USE;