softpipe: remove shadow_ref assert.

I don't think this really buys us anything and TG4 with cubemap arrays
falls over because sampler == 2, but otherwise works fine.

Fixes:
./bin/textureGather fs shadow r  CubeArray repeat

on softpipe with ARB_gpu_shader5 enabled.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/tgsi/tgsi_exec.c

@@ -2274,7 +2274,6 @@ exec_tex(struct tgsi_exec_machine *mach,
          FETCH(&r[last], 0, TGSI_CHAN_W);
       }
       else {
-         assert(shadow_ref != 4);
          FETCH(&r[last], 1, TGSI_CHAN_X);
       }