spl-proc: remove old taskq stats

These had minimal useful information for the admin, didn't work properly
in some places, and knew far too much about taskq internals.

With the new stats available, these should never be needed anymore.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Syneto
Closes #16171