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

module/os/freebsd/zfs/arc_os.c

@@ -160,8 +160,6 @@ arc_prune_task(void *arg)
 {
 	int64_t nr_scan = (intptr_t)arg;
 
-	arc_reduce_target_size(ptob(nr_scan));
-
 #ifndef __ILP32__
 	if (nr_scan > INT_MAX)
 		nr_scan = INT_MAX;