community: Remove the system message count limit for ChatTongyi. (#30192)

## Description
The models in DashScope support multiple SystemMessage. Here is the
[Doc](https://bailian.console.aliyun.com/model_experience_center/text#/model-market/detail/qwen-long?tabKey=sdk),
and the example code on the document page:
```python
import os
from openai import OpenAI

client = OpenAI(
    api_key=os.getenv("DASHSCOPE_API_KEY"),  # 如果您没有配置环境变量，请在此处替换您的API-KEY
    base_url="https://dashscope.aliyuncs.com/compatible-mode/v1",  # 填写DashScope服务base_url
)
# 初始化messages列表
completion = client.chat.completions.create(
    model="qwen-long",
    messages=[
        {'role': 'system', 'content': 'You are a helpful assistant.'},
        # 请将 'file-fe-xxx'替换为您实际对话场景所使用的 file-id。
        {'role': 'system', 'content': 'fileid://file-fe-xxx'},
        {'role': 'user', 'content': '这篇文章讲了什么？'}
    ],
    stream=True,
    stream_options={"include_usage": True}
)

full_content = ""
for chunk in completion:
    if chunk.choices and chunk.choices[0].delta.content:
        # 拼接输出内容
        full_content += chunk.choices[0].delta.content
        print(chunk.model_dump())

print({full_content})
```
Tip: The example code is for OpenAI, but the document said that it also
supports the DataScope API, and I tested it, and it works.
```
Is the Dashscope SDK invocation method compatible?

Yes, the Dashscope SDK remains compatible for model invocation. However, file uploads and file-ID retrieval are currently only supported via the OpenAI SDK. The file-ID obtained through this method is also compatible with Dashscope for model invocation.
```

libs/community/langchain_community/chat_models/tongyi.py

@@ -783,8 +783,6 @@ class ChatTongyi(BaseChatModel):
         ]
         if len(system_message_indices) == 1 and system_message_indices[0] != 0:
             raise ValueError("System message can only be the first message.")
-        elif len(system_message_indices) > 1:
-            raise ValueError("There can be only one system message at most.")
 
         params["messages"] = message_dicts