Remove dependency on RetrieveAssistantAgent for RetrieveChat

[thinkall]

Why are these changes needed?

Current RAG agents RetrieveUserProxyAgent and RetrieveAssistantAgent work together to provide RAG functionality. However, using one single RAG agent would be more convenient for group chat scenarios. At the beginning, the reason to have a RetrieveAssistantAgent instead of any AssistantAgent is that we need RetrieveAssistantAgent to help handle human input "Update Context". Now, we can handle this with process_message_before_send hook.

So, in this PR, we remove dependency on RetrieveAssistantAgent for RetrieveChat.

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