Alerting: Remove transitional notifications API endpoints (#102713)

* Remove transitional notifications API endpoints

* Fix openapi spec

* Fix path test

* Remove API tests related to removed endpoints

pkg/services/ngalert/api/api.go

@@ -181,12 +181,6 @@ func (api *API) RegisterAPIEndpoints(m *metrics.API) {
 		hist:   api.Historian,
 	}), m)
 
-	api.RegisterNotificationsApiEndpoints(NewNotificationsApi(&NotificationSrv{
-		logger:            logger,
-		receiverService:   api.ReceiverService,
-		muteTimingService: api.MuteTimings,
-	}), m)
-
 	if api.FeatureManager.IsEnabledGlobally(featuremgmt.FlagAlertingConversionAPI) {
 		api.RegisterConvertPrometheusApiEndpoints(NewConvertPrometheusApi(
 			NewConvertPrometheusSrv(