drivers/rtc/rtc-wm831x.c: remove unused period IRQ handler

Due to changes in the RTC core the period interrupt is now unused so
delete the code managing it.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>