Remove the gitlab duo chat backdrop

What does this MR do and why?

Remove the gitlab duo chat backdrop

Changelog: changed

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Make sure Gitlab Duo is working on your local setup by following these steps

1. Open the Gitlab Duo chat in the group where you enabled the Ultimate license in the above setup process
2. Click on Help in the left navbar and then click on Ask Gitlab Duo
3. See that there is no backdrop when gitlab chat drawer is opened
4. See that the drawer can be closed only on "close" icon click

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 #417601