lib: sbi_pmu: Remove "event_idx" member from struct sbi_pmu_fw_event

The "event_idx" member of struct sbi_pmu_fw_event is not used
anywhere so let us remove it.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>