routing: stop tracking totalAmount in paymentLifecycle

This commit removes the field `totalAmount` from `paymentLifecycle` and
only reads it from the channeldb payment.

routing/mock_test.go

@@ -758,6 +758,7 @@ func (m *mockControlTower) FetchPayment(phash lntypes.Hash) (
 	// Make a copy of the payment here to avoid data race.
 	p := args.Get(0).(*channeldb.MPPayment)
 	payment := &channeldb.MPPayment{
+		Info:          p.Info,
 		FailureReason: p.FailureReason,
 	}
 	payment.HTLCs = make([]channeldb.HTLCAttempt, len(p.HTLCs))