Remove avatar container from groups home panel

What does this MR do and why?

It removes .avatar_container from app/views/groups/_home_panel.html.haml because it's no longer being used.

Solves #435365 (closed)

Screenshots or screen recordings

before desktop:

after desktop:

before mobile:

after mobile:

How to set up and validate locally

1. Ensure pipelines pass
2. Go to a group and ensure the logos are the same before/after the changes

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.