Remove claude 4.0 (Root Cause Analysis and Troubleshoot Job) feature flags

What does this MR do and why?

We kicked off the following initiative #545117 (closed) in %18.1 to migrate all our Duo features to Claude 4.0. At this point, we have all the features except UP.

The following features still need migration to claude_4_0:

### ❌ Features NOT using claude_4_0:

- **Explain Vulnerability** - Currently uses `claude_3_7` (1.0.1) - [Issue #545697](https://gitlab.com/gitlab-org/gitlab/-/issues/545697)
- **Resolve Vulnerability** - Currently uses `claude_3_7` (1.0.1) - [Issue #545698](https://gitlab.com/gitlab-org/gitlab/-/issues/545698)  
- **Summarize Comments** - Currently uses `claude_3_5` (1.1.0)
- **Categorize Question** - Currently uses `claude_3_5` (1.0.0)

To expedite wrapping up this issue, I've removed our existing feature flags for Duo Code Review, Explain Vulnerability to push through our changes to our docs.

References

Claude Sonnet 4.0 Upgrade (#545117 - closed)

Follow-up from "Add RCA Claude 4 upgrade" (#551885)

Claude 4.0 Sonnet Duo Code Review Rollout Plan (#545502 - closed)

Claude 4.0 Sonnet Explain Vulnerability Rollout... (#545697 - closed) • Neil McCorrison • 18.8 • At risk

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