Alerting: Remove back end logic for supporting KeepLastState (#34242)

* Removed back end logic for supporting KeepLastState

* Map keep_state correctly in migrations

pkg/services/ngalert/api/tooling/definitions/cortex-ruler.go

@@ -279,18 +279,16 @@ func (n *GettableExtendedRuleNode) validate() error {
 type NoDataState string
 
 const (
-	Alerting      NoDataState = "Alerting"
-	NoData        NoDataState = "NoData"
-	KeepLastState NoDataState = "KeepLastState"
-	OK            NoDataState = "OK"
+	Alerting NoDataState = "Alerting"
+	NoData   NoDataState = "NoData"
+	OK       NoDataState = "OK"
 )
 
 // swagger:enum ExecutionErrorState
 type ExecutionErrorState string
 
 const (
-	AlertingErrState      ExecutionErrorState = "Alerting"
-	KeepLastStateErrState ExecutionErrorState = "KeepLastState"
+	AlertingErrState ExecutionErrorState = "Alerting"
 )
 
 // swagger:model