zfs_remove: assert that delete_now case is never true on FreeBSD

That case is specific to Solaris VFS and it would violate pretty
fundamental contracts of FreeBSD VFS.

Discussed with:	pjd
MFC after:	12 days

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c

@@ -1908,6 +1908,9 @@ zfs_remove(vnode_t *dvp, char *name, cred_t *cr, caller_context_t *ct,
 	}
 
 	if (delete_now) {
+#ifdef __FreeBSD__
+		panic("zfs_remove: delete_now branch taken");
+#endif
 		if (xattr_obj_unlinked) {
 			ASSERT3U(xzp->z_links, ==, 2);
 			mutex_enter(&xzp->z_lock);