Remove set but not used variable in ddt.c (#16522)

module/zfs/ddt.c:2612:6: error: variable 'total' set but not used

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>