nak: Remove assert on nir->info.outputs_written

nir->info.outputs_written isn't used for fragment shaders except as an
early out a few lines above this, so we don't rely on this property.

My best guess is that this was intended to check if the information
from nir_gather_info is stale, but dead variables fail the assert
even if the info is up to date.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27595>

src/nouveau/compiler/nak_nir.c

@@ -1043,7 +1043,6 @@ nak_nir_lower_fs_outputs(nir_shader *nir)
 
    nir->num_outputs = 0;
    nir_foreach_shader_out_variable(var, nir) {
-      assert(nir->info.outputs_written & BITFIELD_BIT(var->data.location));
       switch (var->data.location) {
       case FRAG_RESULT_DEPTH:
          assert(var->data.index == 0);