collab: Remove unused `POST /predict_edits` endpoint from LLM service (#23997)

This PR removes the `POST /predict_edits` endpoint from the LLM service,
as it has been superseded by the corresponding endpoint running in
Cloudflare Workers.

All traffic is already being routed to the Cloudflare Workers via the
Workers route, so nothing is hitting this endpoint running in the LLM
service anymore.

You can see the drop off in requests to this endpoint on this graph when
the Workers route was added:

<img width="472" alt="Screenshot 2025-01-30 at 9 18 04 PM"
src="https://github.com/user-attachments/assets/fa60f7c8-2737-4329-88a3-17093bdb5a29"
/>

We also don't use the `fireworks` crate anymore in this repo, so it has
been removed.

Release Notes:

- N/A