vdev_raidz: asize/psize: remove unnecessary var initialisation

It would have been optimised away anyway so it doesn't matter, but it
does make things a little tougher to read.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #17551