stm32/rtc: Don't try to set SubSeconds value on RTC.

The hardware doesn't allow it, instead the value is reset to 255 upon
setting the other calendar/time values.

docs/library/pyb.RTC.rst

@@ -31,7 +31,7 @@ Methods
    
    With no arguments, this method returns an 8-tuple with the current
    date and time.  With 1 argument (being an 8-tuple) it sets the date
-   and time.
+   and time (and ``subseconds`` is reset to 255).
    
    .. only:: port_pyboard
    

ports/stm32/rtc.c

@@ -529,7 +529,6 @@ mp_obj_t pyb_rtc_datetime(size_t n_args, const mp_obj_t *args) {
         time.Hours = mp_obj_get_int(items[4]);
         time.Minutes = mp_obj_get_int(items[5]);
         time.Seconds = mp_obj_get_int(items[6]);
-        time.SubSeconds = rtc_us_to_subsec(mp_obj_get_int(items[7]));
         time.TimeFormat = RTC_HOURFORMAT12_AM;
         time.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
         time.StoreOperation = RTC_STOREOPERATION_SET;