routing/route+multi: remove redundant TotalFees field

Instead get it on demand using method TotalFees().

routing/router_test.go

@@ -199,7 +199,7 @@ func TestFindRoutesWithFeeLimit(t *testing.T) {
 		t.Fatalf("unable to find any routes: %v", err)
 	}
 
-	if route.TotalFees > restrictions.FeeLimit {
+	if route.TotalFees() > restrictions.FeeLimit {
 		t.Fatalf("route exceeded fee limit: %v", spew.Sdump(route))
 	}