Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/250681

* Remove unused methods
  `disallowed_project_visibility_level_description`
  `disallowed_group_visibility_level_description`
  `visibility_level_errors_for_group`

* Move locally used methods to the private scope (for consistency)
  `project_visibility_level_description`
  `group_visibility_level_description`
  `project_visibility_icon_description`
  `group_visibility_icon_description`