[fea-rs] Don't write lookups that are not referenced by a feature

Not sure how this bug has hung around for so long, or if I'm only seeing this in some funny combination of conditions? In any case it is exhibited in at least one font on crater.

```
python3 -m ttx_diff 'https://github.com/notofonts/bhaiksuki?570db5a125#sources/NotoSansBhaiksuki.designspace'
```

Start by running the provided ttx_diff command against the NotoSansBhaiksuki.designspace source and inspect which unreferenced lookups are written. Trace the fea-rs lookup-generation path to identify the relevant entry point. Done means the output no longer writes lookups that are not referenced by a feature, while preserving referenced lookups.

Written by the indexing model from the issue text.