Remove ai_catalog_agents feature flag

What does this MR do and why?

Remove ai_catalog_agents feature flag from frontend and backend.

Also remove "Enable in project" as it won't ever be shown with the current logic.

References

Related to #580685

Screenshots or screen recordings

NA

How to set up and validate locally

1. Prerequisite: You have a Duo workflow-enabled project.
2. Enable the feature flags global_ai_catalog.
3. Go to Explore > AI Catalog > Agents. For example, https://gdk.test:3443/explore/ai-catalog/agents.
   1. Create an agent. You should be redirected to an agent page.
   2. Select Enable in group. Select Group and choose a Duo-enabled group.
4. Go to Group > Automate > Agents. For example, https://gdk.test:3443/groups/gitlab-duo/-/automate/agents.
   1. Verify that the agent is enabled for the group.
   2. Select the agent to view the show page.
5. Go to Project > Automate > Agents. For example, https://gdk.test:3443/gitlab-duo/test/-/automate/agents.
   1. Select Enable agent in project. Select the agent added to the group.
   2. Select the agent to view the show page.

MR acceptance checklist

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