RTC: Cleanup rtc_class_ops->irq_set_freq()

With the generic rtc code now emulating PIE mode irqs via an
hrtimer, no one calls the rtc_class_ops->irq_set_freq call.

This patch removes the hook and deletes the driver functions
if no one else calls them.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Alessandro Zummo <a.zummo@towertech.it>
CC: Marcelo Roberto Jimenez <mroberto@cpti.cetuc.puc-rio.br>
CC: rtc-linux@googlegroups.com
Signed-off-by: John Stultz <john.stultz@linaro.org>