Remove FF allow_duo_base_access from backend

What does this MR do and why?

After the rollout of the FF, we want to remove the usage from the backend. This merge request should get merged and rolled out to production after the merge request !191509 (merged) reached production. With the previous merge request we remove all usages from of the FF in the frontend and so we can remove it as well from the backend.

This merge request touches 6 locations.

• ee/app/helpers/admin/application_settings_helper.rb

  • Was used for pages like /admin/gitlab_duo

• ee/app/helpers/ee/groups/settings_helper.rb

  • Was used for pages like /groups/<group>/-/settings

• ee/app/helpers/ee/users/callouts_helper.rb

  • Is used to decide if the Duo Core activation banner gets shown for SM.

    

• ee/app/helpers/ee/users/group_callouts_helper.rb

  

• ee/app/helpers/ee/groups_helper.rb

  • Was used on pages like /groups/<group>/-/settings/gitlab_duo

• ee/app/policies/ee/global_policy.rb

  • For the global policy access_duo_core_features

References

• Rollout issue https://gitlab.com/gitlab-org/gitlab/-/issues/536311+
• Should be deployed after the frontend cleanup is online Remove FF allow_duo_base_access from frontend (!191509 - merged)

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Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/544848