llava-cli : refactor to use sampling library (#4669)

This change makes it possible to use flags like `--grammar` when using
the `llava-cli` program. The rest is just code cleanup deleting a long
standing TODO comment.

This change also ensures that logging information is emitted to stderr
which helps the `llava-cli` command be more friendly to shell scripts.

See Mozilla-Ocho/llamafile@1cd334f