### What problem does this PR solve?
This pull request makes a small but important fix to how streaming
requests are handled in the `completion` endpoint of
`conversation_app.py`. The main change ensures that the `stream`
argument is not passed twice, which could cause errors.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
Signed-off-by: Jin Hai <haijin.chn@gmail.com>