groq[minor]: remove default model (#30341)

The default model for `ChatGroq`, `"mixtral-8x7b-32768"`, is being
retired on March 20, 2025. Here we remove the default, such that model
names must be explicitly specified (being explicit is a good practice
here, and avoids the need for breaking changes down the line). This
change will be released in a minor version bump to 0.3.

This follows https://github.com/langchain-ai/langchain/pull/30161
(released in version 0.2.5), where we began generating warnings to this
effect.

![Screenshot 2025-03-18 at 10 33
27 AM](https://github.com/user-attachments/assets/f1e4b302-c62a-43b0-aa86-eaf9271e86cb)

libs/partners/groq/tests/unit_tests/test_chat_models.py

@@ -2,7 +2,6 @@
 
 import json
 import os
-import warnings
 from typing import Any
 from unittest.mock import AsyncMock, MagicMock, patch
 
@@ -280,23 +279,3 @@ def test_groq_serialization() -> None:
 
     # Ensure a None was preserved
     assert llm.groq_api_base == llm2.groq_api_base
-
-
-def test_groq_warns_default_model() -> None:
-    """Test that a warning is raised if a default model is used."""
-
-    # Delete this test in 0.3 release, when the default model is removed.
-
-    # Test no warning if model is specified
-    with warnings.catch_warnings():
-        warnings.simplefilter("error")
-        ChatGroq(model="foo")
-
-    # Test warns if default model is used
-    with pytest.warns(match="default model"):
-        ChatGroq()
-
-    # Test only warns once
-    with warnings.catch_warnings():
-        warnings.simplefilter("error")
-        ChatGroq()