fireworks[minor]: remove default model and temperature (#30965)

`mixtral-8x-7b-instruct` was recently retired from Fireworks Serverless.

Here we remove the default model altogether, so that the model must be
explicitly specified on init:
```python
ChatFireworks(model="accounts/fireworks/models/llama-v3p1-70b-instruct")  # for example
```

We also set a null default for `temperature`, which previously defaulted
to 0.0. This parameter will no longer be included in request payloads
unless it is explicitly provided.

libs/partners/fireworks/README.md

@@ -39,7 +39,7 @@ import os
 
 # Initialize a Fireworks model
 llm = Fireworks(
-    model="accounts/fireworks/models/mixtral-8x7b-instruct",
+    model="accounts/fireworks/models/llama-v3p1-8b-instruct",
     base_url="https://api.fireworks.ai/inference/v1/completions",
 )
 ```