rtc: mt6397: drop unused defines

RTC_NUM_YEARS has never been used, the other defines are not used since
commit 34bbdc12 ("rtc: mt6359: Add RTC hardware range and add support
for start-year")

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20250303223637.1135362-1-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>