hrtimer: Remove stale comment on curr_timer

curr_timer doesn't resident in struct hrtimer_cpu_base anymore.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
LKML-Reference: <1287892253-2587-1-git-send-email-yong.zhang0@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>