Remove clean slash command

What does this MR do and why?

Integration branch for:

DuoChat: Remove undocumented /clean command (gitlab-ui#2788 - closed)

fix(DuoChat): Remove the 'clean' slash command (gitlab-ui!4415 - merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Checkout this branch locally, run bundle install and yarn, then start your GDK
2. Create some conversation with Duo Chat
3. Execute the /clean command and a response from the model will occur
4. Execute the /clear command the chat will return to it's initial UI

Screenshots or screen recordings

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

Before	After
NOTremoveCleanSlashCommand_test	cleanSlashCommand_removedTest