add spec and changelog

changelogs/unreleased/fix-orphan-notification-settings.yml

+---
+title: Removed orphaned notification settings without a namespace
+merge_request:
+author:

spec/services/groups/destroy_service_spec.rb

@@ -7,6 +7,7 @@ describe Groups::DestroyService, services: true do
   let!(:group)        { create(:group) }
   let!(:nested_group) { create(:group, parent: group) }
   let!(:project)      { create(:empty_project, namespace: group) }
+  let!(:notification_setting) { create(:notification_setting, source: group)}
   let!(:gitlab_shell) { Gitlab::Shell.new }
   let!(:remove_path)  { group.path + "+#{group.id}+deleted" }
 
@@ -23,6 +24,7 @@ describe Groups::DestroyService, services: true do
       it { expect(Group.unscoped.all).not_to include(group) }
       it { expect(Group.unscoped.all).not_to include(nested_group) }
       it { expect(Project.unscoped.all).not_to include(project) }
+      it { expect(NotificationSetting.unscoped.all).not_to include(notification_setting) }
     end
 
     context 'file system' do