rtc: vt8500: Fix build error & cleanup rtc_class_ops->update_irq_enable()

Now that the generic code handles UIE mode irqs via periodic
alarm interrupts, no one calls the
rtc_class_ops->update_irq_enable() method anymore.

Further the rtc_class_ops doesn't have a update_irq_enable element
anymore, so this causes a build error.

This patch removes the driver hooks and implementations of
update_irq_enable and the associated setup.

[wsa: updated commit-message and removed update_irq_enable-function, too]
[jstultz: improve commit message, clarifying build issue]
Signed-off-by: Alexey Charkov <alchark@gmail.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>