rtc: ds1307: remove redundant code in rtc_reset

As of commit 1a1fa240 ("rtc: Set valid date after reset") the
command "date reset" will set the date/time to 2000-01-01 0:00:00 after
calling rtc_reset(). This means that the ds1307 implementation of
rtc_reset() doesn't need to call rtc_set().

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>