rtc: pcf85063: remove useless century handling

pcf85063_get_datetime() tries to handle a century bit but that bit is not
documented and the final value is never used anywhere else in the kernel.

Reviewed-by: Juergen Borleis <jbe@pengutronix.de>
Tested-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>