genai: fix buggy test case for array with nullable anyof

- An `items` field in JSON Schema can't have `items` as its direct child

libs/genai/tests/unit_tests/test_function_utils.py

@@ -84,11 +84,6 @@ def possibly_none_list(
     # Convert to OpenAI tool
     oai_tool = convert_to_openai_tool(possibly_none_list)
 
-    # Manually assign the 'items' type in the parameters
-    oai_tool["function"]["parameters"]["properties"]["items"]["items"] = {
-        "type": "string"
-    }
-
     # Convert to GenAI, then to dict
     genai_tool = convert_to_genai_function_declarations([oai_tool])
     genai_tool_dict = tool_to_dict(genai_tool)