Replace `data-confirm` modal with `GlModal` in `app/views/admin/topics/_form.html.haml`

What does this MR do and why?

This MR replaces the topic remove avatar confirm dialog with a gitlab-ui modal.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Browse to admin > overview > topics
• Add a topic (if one doesn't exists)
• Edit the topic
• Add an avatar
• Save changes
• Remove the avatar

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.

Related to #344184 (closed)