Remove feature flag: FF_ENABLE_DOMAIN_REDIRECT

Steps to perform:

• Remove feature flag: FF_ENABLE_DOMAIN_REDIRECT: !1036 (merged)
• Remove the infra code added to enable feature flag in all environment.
  • Staging: gitlab-com/gl-infra/k8s-workloads/gitlab-com!3795 (merged)
  • Production: gitlab-com/gl-infra/k8s-workloads/gitlab-com!3817 (merged)

Reference issue: #1087 (closed)