nir: Remove nir_alu_dest::write_mask

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24432>

src/compiler/nir/tests/range_analysis_tests.cpp

@@ -61,7 +61,6 @@ ssa_def_bits_used_test::build_alu_instr(nir_op op,
    if (alu == NULL)
       return NULL;
 
-   alu->dest.write_mask = 1;
    alu->dest.dest.ssa.num_components = 1;
 
    return alu;