[BUG] lighteval does not support openai endpoints but says it does

## Describe the bug
In the README and when running help, there are references to openai endpoint support, but it appears to no longer exist.
```
$ lighteval endpoint --help

Usage: lighteval endpoint [OPTIONS] COMMAND [ARGS]...

Evaluate models using some endpoint (tgi, inference endpoint, openai) as backend.

╭─ Options ────────────────────────────────────────────────────────────────────────────────────────────╮
│ --help Show this message and exit. │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────╯
╭─ Evaluation Backends ────────────────────────────────────────────────────────────────────────────────╮
│ inference-endpoint Evaluate models using inference-endpoints as backend. │
│ tgi Evaluate models using TGI as backend. │
│ litellm Evaluate models using LiteLLM as backend. │
│ inference-providers Evaluate models using LiteLLM as backend. │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────╯
```

```
$ lighteval endpoint openai --help
Usage: lighteval endpoint [OPTIONS] COMMAND [ARGS]...
Try 'lighteval endpoint --help' for help.
╭─ Error ──────────────────────────────────────────────────────────────────────────────────────────────╮
│ No such command 'openai'. │
╰──────────────────────────────────────────────────────────────────────────────────────────────────────╯
```

## Expected behavior
If the openai/openai-compatible support was removed, please remove it from your documentation or indicate clearly that it is no longer supported.