Remove unimplemented virus scanning hooks

Reviewed-by: Adam Moss <c@yotes.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11972

include/os/freebsd/zfs/sys/zfs_vfsops_os.h

@@ -98,7 +98,6 @@ struct zfsvfs {
 	struct zfsctl_root	*z_ctldir;	/* .zfs directory pointer */
 	boolean_t	z_show_ctldir;	/* expose .zfs in the root dir */
 	boolean_t	z_issnap;	/* true if this is a snapshot */
-	boolean_t	z_vscan;	/* virus scan on/off */
 	boolean_t	z_use_fuids;	/* version allows fuids */
 	boolean_t	z_replay;	/* set during ZIL replay */
 	boolean_t	z_use_sa;	/* version allow system attributes */