PM: runtime: Remove unsued extern declaration of pm_runtime_update_max_time_suspended()

Commit 76e267d8 ("PM / Runtime: Remove device fields related to
suspend time, v2") left behind this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>