[9fans] compiler warnings for libavl/avl.c

[EBo]

The following two subroutines are defined and not used anywhere.  Should they
redefined as void or possibly removed?

  EBo --

================

static void
walkavl(Avl *t, void (*f)(Avl*, void*), void *v)
{
        if(t == nil)
                return;
        walkavl(t->n[0], f, v);
        f(t, v);
        walkavl(t->n[1], f, v);
}

static void
checkparents(Avl *a, Avl *p)
{
        if(a == nil)
                return;
        if(a->p != p)
                print("bad parent\n");
        checkparents(a->n[0], a);
        checkparents(a->n[1], a);
}