zink: do not leave needless shader temps around

This used to not matter, but since we started emitting shader-temps
properly, this causes issues where we end up with samplers and images as
shader-temps. That causes asserts while emitting them.

So let's remove the unused vars as well.

This fixes a piglit regression that somehow went unnoticed on CI.

Fixes: 85964945e7 ("zink: emit vars with nir_var_shader_temp mode")
Tested-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20227>

src/gallium/drivers/zink/zink_compiler.c

@@ -2497,6 +2497,7 @@ prune_io(nir_shader *nir)
       if (!find_var_deref(nir, var))
          var->data.mode = nir_var_shader_temp;
    }
+   NIR_PASS_V(nir, nir_remove_dead_variables, nir_var_shader_temp, NULL);
 }
 
 VkShaderModule