[pack] Removing Multi-Language feature flag

[siddharth-ms]

Issue describing the changes in this PR

Feature #8538 was under a feature flag EnableMultiLanguageWorker for A/B. We have now removed it to make it default for all LA.

Pull request checklist

• [X] My changes do not require documentation changes
  • [ ] Otherwise: Documentation issue linked to PR
• [X] My changes should not be added to the release notes for the next release
  • [ ] Otherwise: I've added my notes to release_notes.md
• [X] My changes do not need to be backported to a previous version
  • [ ] Otherwise: Backport tracked by issue/PR #issue_or_pr
• [X] My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• [X] I have added all required tests (Unit tests, E2E tests)