channeldb: remove redundant MPPaymentCreationInfo struct

MPPaymentCreationInfo and PaymentCreationInfo are identical except for
the naming of CreationTime/CreationDate.

routing/control_tower_test.go

@@ -311,7 +311,7 @@ func genInfo() (*channeldb.PaymentCreationInfo, *channeldb.PaymentAttemptInfo,
 	return &channeldb.PaymentCreationInfo{
 			PaymentHash:    rhash,
 			Value:          1,
-			CreationDate:   time.Unix(time.Now().Unix(), 0),
+			CreationTime:   time.Unix(time.Now().Unix(), 0),
 			PaymentRequest: []byte("hola"),
 		},
 		&channeldb.PaymentAttemptInfo{

routing/router.go

@@ -1680,7 +1680,7 @@ func (r *ChannelRouter) preparePayment(payment *LightningPayment) (
 	info := &channeldb.PaymentCreationInfo{
 		PaymentHash:    payment.PaymentHash,
 		Value:          payment.Amount,
-		CreationDate:   time.Now(),
+		CreationTime:   time.Now(),
 		PaymentRequest: payment.PaymentRequest,
 	}
 
@@ -1709,7 +1709,7 @@ func (r *ChannelRouter) SendToRoute(hash lntypes.Hash, route *route.Route) (
 	info := &channeldb.PaymentCreationInfo{
 		PaymentHash:    hash,
 		Value:          amt,
-		CreationDate:   time.Now(),
+		CreationTime:   time.Now(),
 		PaymentRequest: nil,
 	}