Autocomplete: Remove top_k when using temperature (#2178)

According to the Fireworks API docs:

> “We generally recommend altering this or `temperature` but not both.”

We previously even had an issue that was not passing `top_p` forward
properly: https://github.com/sourcegraph/sourcegraph/pull/58820

This PR removes top_p from the client. I ran a bunch of direct API
requests to Fireworks and its seems like the `top_p` value makes no
difference when a temperature is set anyways.

## Test plan

Tested via CURL commands:

```
curl 'https://api.fireworks.ai/inference/v1/completions' \
  -H 'authority: api.fireworks.ai' \
  -H 'accept: text/event-stream' \
  -H 'authorization: Bearer TOKEN HERE' \
  -H 'content-type: application/json' \
  --data-raw '{"model":"accounts/fireworks/models/starcoder-16b-w8a16","prompt":"<filename>write.ts<fim_prefix>// \nfunction writeCurrentDateToFile() {\n  <fim_suffix>\n}<fim_middle>","n":1,"max_tokens":500,"temperature":0.1,"top_p":1}' \
```

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