UDFs with non Stan-compatible return types should not be written out

In the models generated at https://nsiccha.github.io/StanBlocks.jl/slic/isba-2024.html, UDFs such as 

```stan
func broadcasted_getindex_logit(int i) {
    return logit;
}
```

are written out, but Stan of course does not accept this. The fix would be to not write these functions out to the Stan code.