vkd3d-shader/hlsl: Cleanup parse_attribute_list structs (valgrind).

This leak is unrelated to !786 (merged), but I found it while reviewing it.

Before this, shaders with attribute lists can make valgrind report the problem:

float4 main() : sv_target
{
    float ret = 0.0;

    [unroll(4)] for (int i = 0; i < 10; ++i)
    {
        ret += 1.0;
    }
    return ret;
}