rtc: stm32: drop unused module alias

The driver only support OF probe so drop the unused platform module
alias.

Fixes: 4e64350f ("rtc: add STM32 RTC driver")
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250423130318.31244-8-johan+linaro@kernel.org


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