Remove LanguageServer Web IDE feature flag

MR: Pending

Description

Once the Language Server is production-ready on the Web IDE– the feature flag is fully rolled out and removed in the Rails app, we can remove the corresponding Web IDE feature flag.

Acceptance Criteria

• Upgrade the browser VSCode extension to include changes from: Clean up "gitlab.featureFlags.languageServerWeb... (gitlab-vscode-extension#1403 - closed).
• Remove language server Web IDE feature flag changes introduced in: Add "LanguageServer" feature flag to Web IDE (#387 - closed)
• Release Web IDE