nir: fix bit of cargo-culting in lower_idiv

I guess I was looking too much at how lower_system_values worked when
writing lower_idiv.

Since ttn wasn't emitting load_var for sysvals and the only drivers
using lower_idiv were using ttn, I think nothing was broken as a result.
But might as well fix this before it becomes a problem.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/glsl/nir/nir_lower_idiv.c

@@ -152,6 +152,4 @@ nir_lower_idiv(nir_shader *shader)
       if (overload->impl)
          convert_impl(overload->impl);
    }
-
-   exec_list_make_empty(&shader->system_values);
 }