MN10300: Delete idle_timestamp from irq_cpustat_t

Delete idle_timestamp from irq_cpustat_t as it's an unread relic.

Signed-off-by: Akira Takeuchi <takeuchi.akr@jp.panasonic.com>
Signed-off-by: Kiyoshi Owada <owada.kiyoshi@jp.panasonic.com>
Signed-off-by: David Howells <dhowells@redhat.com>