FreeBSD: Remove extra arc_reduce_target_size() call

Remove arc_reduce_target_size() call from arc_prune_task(). The idea
of arc_prune_task() is to remove external references on ARC metadata,
such as vnodes. Since arc_prune_async() is called only from ARC itself,
it makes no sense to create a parasitic loop between ARC eviction and
the pruning, treatening to drop ARC to its minimum. I can't guess why
it was added as part of FreeBSD to OpenZFS integration.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14639