graw: remove pipe_vertex_buffer::max_index

This is a follow-up to commit cdca3c58aa.

src/gallium/tests/graw/quad-sample.c

@@ -99,7 +99,6 @@ static void set_vertices( void )
 
 
    vbuf.stride = sizeof( struct vertex );
-   vbuf.max_index = sizeof(vertices) / vbuf.stride;
    vbuf.buffer_offset = 0;
    vbuf.buffer = screen->user_buffer_create(screen,
                                             vertices,