Cleanup feature flag for ordering by semver in Terraform modules

What does this MR do and why?

Clean-up the order_by_semver_in_terraform_modules feature flag to support sorting by SemVer in the Terraform Registry.

The feature flag has been globally enabled on gitlab.com for a week now without any issues, so we can go ahead and remove it from the codebase.

References

• [FF] `order_by_semver_in_terraform_modules` -- (#562699 - closed) • Moaz Khalifa • 18.4
• Terraform registry: "Latest version for a speci... (#474746 - closed) • Moaz Khalifa • 18.4

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Related to #562699 (closed)