feat: remove ai_duo_agent_platform_ga_rollout feature flag

Description

This MR implements #2233 (closed).

This MR needs gitlab-org/editor-extensions/gitlab-lsp!2880 (merged) to be merged and published, and the LS version to be updated.

Related Issues

Resolves #2233 (closed)

How has this been tested?

1. Run vscode extension
2. Check that agentic duo chat is available and does not have a beta label

• If src/browser or src/common has been modified, please consider interoperability with the Web IDE. See Running the Extension in WebIDE.
• Consider an end-to-end test for significant new features that aren't covered by integration tests.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes