feat(duo_chat_threads): remove useless avatar from chat threads thread box

What does this MR do?

We removed the circular avatar that appeared next to each conversation in the Duo Chat history list. Each entry now leads directly with the conversation title.

• The avatar was meaningless. It was an auto-generated placeholder based on the chat title, not a real person or agent. It implied identity without conveying any actual information.
• Less visual noise. Removing it lets the eye go straight to the title, which is the meaningful differentiator in the list.
• More space for text. Dropping the fixed-width avatar gives titles and metadata more room, which helps in the narrow chat panel.
• Cleaner and more scannable. Titles now align consistently, making the history faster to scan.

A simpler, more scannable chat history list that prioritizes the conversation title and drops a purely decorative element.

Screenshots or screen recordings

Before	After

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