Clean up linked_work_items feature flag

...ts/javascripts/work_items/components/work_item_detail.vue

app/controllers/groups/work_items_controller.rb

app/controllers/groups_controller.rb

app/controllers/projects/incidents_controller.rb

app/controllers/projects/issues_controller.rb

app/controllers/projects/work_items_controller.rb

app/controllers/projects_controller.rb

app/graphql/mutations/work_items/linked_items/base.rb

app/graphql/resolvers/work_items/linked_items_resolver.rb

app/graphql/types/work_items/widgets/linked_items_type.rb

app/models/group.rb

app/models/project.rb

config/feature_flags/development/linked_work_items.yml

doc/api/graphql/reference/index.md

doc/architecture/blueprints/work_items/index.md

doc/user/okrs.md

doc/user/tasks.md

.../graphql/ee/types/work_items/widgets/linked_items_type.rb

ee/spec/requests/api/graphql/work_item_spec.rb

spec/features/projects/work_items/linked_work_items_spec.rb