rtc: ds1511: remove unused function

Fix the following clang warning:

drivers/rtc/rtc-ds1511.c:108:1: warning: unused function
'rtc_write_alarm' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1618475821-102974-1-git-send-email-jiapeng.chong@linux.alibaba.com