rtc: meson: remove useless rtc_nvmem_unregister call

rtc_nvmem_unregister() is called on rtc_device release so it is not
necessary to call it from the driver.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>