rtc: class: remove bogus documentation

rtc_device_unregister is gone since commit fdcfd854 ("rtc: rework
rtc_register_device() resource management"). Remove its documentation.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210202112219.3610853-2-alexandre.belloni@bootlin.com