FreeBSD: Prune some unneeded definitions

IS_XATTRDIR is never used.
v_count is only used in two places, one immediately followed by the
use of the real name, v_usecount.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #11973

include/os/freebsd/spl/sys/vnode.h

@@ -66,13 +66,8 @@ typedef	struct vop_vector	vnodeops_t;
 #define	vop_fid_args	vop_vptofh_args
 #define	a_fid		a_fhp
 
-#define	IS_XATTRDIR(dvp)	(0)
-
-#define	v_count	v_usecount
-
 #define	rootvfs		(rootvnode == NULL ? NULL : rootvnode->v_mount)
 
-
 #ifndef IN_BASE
 static __inline int
 vn_is_readonly(vnode_t *vp)