[vscode] Rollback ai_duo_agent_platform_ga_rollout Feature flag related logic

Problem

Once the ai_duo_agent_platform_ga_rollout feature flag is removed from GitLab, vscode will fallback to assuming the flag is set to false and would switch the label of the Duo Agentic Chat webview to the previous Beta label.

Solution

We should remove the logic that relies on that flag (see !3047 (diffs)) and remove the Beta label from the webview name.

Related

See this MR thread: gitlab!219679 (comment 3086383711)