Remove the "Ling 2.6 Flash Free" model from the OpenCode provider's
built-in
model list, as it is no longer available via the API. Attempting to use
it
returns the following error:

```
invalid request format to OpenCode's API: {"error":{"message":"Error from provider: Ling-2.6-flash is no longer available as a free model. It has transitioned to a paid model. Continue using it here: https://openrouter.ai/inclusionai/ling-2.6-flash","code":404},"user_id":"user_2..."}
```

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Removed Ling 2.6 Flash Free model from the OpenCode provider