delete the default model value from langchain and discard the need fo… (#24915)

- description: I remove the limitation of mandatory existence of
`QIANFAN_AK` and default model name which langchain uses cause there is
already a default model nama underlying `qianfan` SDK powering langchain
component.

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/community/tests/integration_tests/chat_models/test_qianfan_endpoint.py

@@ -306,7 +306,10 @@ def test_functions_call() -> None:
 
 def test_rate_limit() -> None:
     chat = QianfanChatEndpoint(model="ERNIE-Bot", init_kwargs={"query_per_second": 2})  # type: ignore[call-arg]
-    assert chat.client._client._rate_limiter._sync_limiter._query_per_second == 2
+    assert (
+        chat.client._client._rate_limiter._internal_qps_rate_limiter._sync_limiter._query_per_second
+        == 1.8
+    )
     responses = chat.batch(
         [
             [HumanMessage(content="Hello")],