Remove feature flag FF_ENABLE_DOMAIN_REDIRECT

What does this MR do?

FF_ENABLE_DOMAIN_REDIRECT is enabled in prod since %16.8 and enabled by default since %16.9 and we have not seen any major issues reported for the feature.

Hence we are removing the feature flag: FF_ENABLE_DOMAIN_REDIRECT.

Related: #1097 (closed)

Changelog: other

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines