fix: No history message attached when for gemini-pro-vision

[fengzai6]

选中gemini-pro-vision的时候，会强制将当前对话设置中附带历史消息为0，这样就不会出现错误：Multiturn chat is not enabled for models/gemini-pro-vision

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[H0llyW00dzZ]

No, the reason is that gemini-pro-vision currently does not support multi-chat like gpt-4-vision-preview does. It might be available in the future, so it's better to maintain the current state rather than refactoring.