Alerting: Remove Multiorg alertmangaer from the top-level services (#38506)

Removes the registration of the multi-org alertmanager, it is started as a subservice of ngalerting.

pkg/api/api.go

@@ -385,7 +385,7 @@ func (hs *HTTPServer) registerRoutes() {
 		})
 
 		apiRoute.Get("/alert-notifiers", reqEditorRole, routing.Wrap(
-			GetAlertNotifiers(hs.MultiOrgAlertmanager != nil && hs.Cfg.IsNgAlertEnabled())),
+			GetAlertNotifiers(hs.Cfg.IsNgAlertEnabled())),
 		)
 
 		apiRoute.Group("/alert-notifications", func(alertNotifications routing.RouteRegister) {