radeonsi: remove indirection when loading position at the end for NGG culling

If we store the position into LDS after we know the new thread ID,
we don't need to remember the old thread ID.

The culling code only needs W, X/W, Y/W, so we have to keep those.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7172>

src/gallium/drivers/radeonsi/si_shader_internal.h

@@ -109,7 +109,6 @@ struct si_shader_context {
     */
    struct ac_arg vs_state_bits;
    struct ac_arg vs_blit_inputs;
-   struct ac_arg ngg_old_thread_id; /* generated by the NGG cull shader */
    /* HW VS */
    struct ac_arg streamout_config;
    struct ac_arg streamout_write_index;