remove conflicting default for tool prompt format in chat completion (#742)

# What does this PR do?
We are setting a default value of json for tool prompt format, which
conflicts with llama 3.2/3.3 models since they use python list. This PR
changes the defaults to None and in the code, we infer default based on
the model.

Addresses: #695 

Tests:
❯ LLAMA_STACK_BASE_URL=http://localhost:5000 pytest -v
tests/client-sdk/inference/test_inference.py -k
"test_text_chat_completion"

 pytest llama_stack/providers/tests/inference/test_prompt_adapter.py

llama_stack/distribution/routers/routing_tables.py

@@ -523,7 +523,6 @@ class ToolGroupsRoutingTable(CommonRoutingTableImpl, ToolGroups):
                     description=tool_def.description or "",
                     parameters=tool_def.parameters or [],
                     provider_id=provider_id,
-                    tool_prompt_format=tool_def.tool_prompt_format,
                     provider_resource_id=tool_def.name,
                     metadata=tool_def.metadata,
                     tool_host=tool_host,