fusefs: fix a dead store in fuse_vnop_advlock

kevans actually caught this in the original review and I fixed it, but
then I committed an older copy of the branch.  Whoops.

Reported by:	kevans
MFC after:	13 days
MFC with:	929acdb19a
Differential Revision:	https://reviews.freebsd.org/D29031

sys/fs/fuse/fuse_vnops.c

@@ -444,7 +444,6 @@ fuse_vnop_advlock(struct vop_advlock_args *ap)
 		break;
 	case F_UNLCK:
 		op = FUSE_SETLK;
-		flags |= F_UNLCK;
 		break;
 	default:
 		return EINVAL;