rtc: Remove hpet_rtc_dropped_irq()

hpet_rtc_dropped_irq() has been unused since
commit f52ef24b ("rtc/alpha: remove legacy rtc driver")

Remove it in rtc, and x86 hpet code.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20241215022356.181625-1-linux@treblig.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>