rtc: max77686: Remove unused code to read in 12-hour mode

The MAX77714 RTC chip is explicitly set to 24-hour mode in
max77686_rtc_probe() -> max77686_rtc_init_reg() and never changed back to
12-hour mode. Accordingly info->rtc_24hr_mode is set to 1 in the same place
and never modified later, so it is de facto a constant. Yet there is code
to read 12-hour time, which is unreachable.

Remove the unused variable, the unreachable code to manage 12-hour mode and
the defines that become unused due to the above changes.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>