hrtimer: Remove the comment about not used HRTIMER_SOFTIRQ

The softirq `HRTIMER_SOFTIRQ' was not used since commit c6eb3f70
("hrtimer: Get rid of hrtimer softirq").

But it got used again, beginning with commit 5da70160 ("hrtimer:
Implement support for softirq based hrtimers"), which did not remove the
comment. Remove it now.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191107091924.13410-1-bigeasy@linutronix.de