FreeBSD: Don't force xattr mount option

The kernel will use the xattr property by default when not overridden
by a mount option.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11997