Remove Duo Custom Rules Feature Flag

Tomas Vikrequested to merge
tv/2025-07/remove-custom-rules-ff into master

What does this MR do and why?

We introduced the flag in !195829 (merged)

And turned it on by default in !196688 (merged)

This is the rollout issue #551804 (closed)

Now that it's been released to self-managed, let's remove it.

How I tested it locally

  1. Set up your GDK with AI Gateway to use Agentic chat (I did it this way gitlab-org/editor-extensions/meta#224 (closed))
  2. Add root PAT to your GitLab VS Code Extension
  3. Open the project with Duo Agentic Chat enabled ( I use http://gdk.test:3000/gitlab-duo/test)
  4. add a rules file (.gitlab/duo/chat-rules.md) to your project https://docs.gitlab.com/user/gitlab_duo_chat/agentic_chat/#create-custom-rules ( I put "talk like a pirate" there)
  5. see the agentic chat follow the rules
Edited by Tomas Vik

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