Remove code related to object hierarchy in MySQL

app/controllers/application_controller.rb

app/controllers/concerns/group_tree.rb

app/finders/group_descendants_finder.rb

app/graphql/types/group_type.rb

app/helpers/groups_helper.rb

app/models/concerns/descendant.rb

app/models/group.rb

app/models/namespace.rb

app/policies/group_policy.rb

app/services/groups/nested_create_service.rb

app/services/groups/transfer_service.rb

app/services/members/destroy_service.rb

app/services/users/refresh_authorized_projects_service.rb

app/views/groups/settings/_advanced.html.haml

ee/app/assets/javascripts/epic/components/epic_body.vue

ee/app/assets/javascripts/epic/store/state.js

ee/app/controllers/groups/epic_links_controller.rb

ee/app/helpers/ee/issuables_helper.rb

ee/app/models/ee/epic.rb

ee/lib/ee/gitlab/quick_actions/epic_actions.rb