dt-bindings: rtc: ds3231: Remove text binding

The "maxim,ds3231" compatible is described in the rtc-ds1307.yaml, so
there is no need to keep the text bindings version.

Remove the maxim,ds3231.txt file in favor of the rtc-ds1307.yaml binding.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230902134407.2589099-1-festevam@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>