partners(mistralai): Removing unused variable in completion request (using tool_calls or content) (#21201)

This PR fixes #21196.

The error was occurring when calling chat completion API with a chat
history. Indeed, the Mistral API does not accept both `content` and
`tool_calls` in the same body.

This PR removes one of theses variables depending on the necessity.

---------

Co-authored-by: Maxime Perrin <mperrin@doing.fr>
Co-authored-by: Chester Curme <chester.curme@gmail.com>

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

@@ -55,7 +55,7 @@ def test_mistralai_initialization() -> None:
         ),
         (
             AIMessage(content="Hello"),
-            dict(role="assistant", content="Hello", tool_calls=[]),
+            dict(role="assistant", content="Hello"),
         ),
         (
             ChatMessage(role="assistant", content="Hello"),