Remove archive_group feature flag from policies

What does this MR do and why?

This commit removes the archive_group feature flag checks from policy files while keeping the flag in the service layer. Groups that are already archived will continue to behave as archived with restricted permissions regardless of the feature flag state.

The feature flag remains in the service to control whether users can archive new groups. When disabled, users cannot archive additional groups, but existing archived groups maintain their archived state and permission restrictions.

References

How to set up and validate locally

1. Create a new group with a new project inside it.
2. Enable the archive_group feature flag: Feature.enable(:archive_group)
3. Switch to the master branch and archive the group:
   • Navigate to Settings → General → Advanced
   • Archive the group
4. Disable the feature flag in Rails console: Feature.disable(:archive_group)
5. Go to the project home page and confirm you can create a new file using the + button
6. Switch to this branch and verify the fix:
   • The + button should no longer appear (preventing file creation in archived groups)

MR acceptance checklist

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Related to #576735 (closed)