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{