Fix stack build warning for dex executable target. (#446)
Fix stack build warning for dex executable target. (#446)

Remove `-threaded` option to fix warning:
```
Warning: 'ghc-options: -threaded' has no effect for libraries. It should only
be used for executables.
```
google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang