rtc: remove irq_task from kerneldoc

Stale mentions of irq_task are left in the kerneldoc after its removal.
Remove them.

There is still one indirect mention left but commit 3c8bb90e

 ("rtc: Fix
hrtimer deadlock") can probably be reverted now.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>