account for changed itineraries once landmarks are marked as done or deleted

frontend/lib/structs/trip.dart

@@ -75,8 +75,16 @@ class Trip with ChangeNotifier {
     notifyListeners();
   }
 
-  void removeLandmark(Landmark landmark) {
+  void removeLandmark (Landmark landmark) async {
+    Landmark? previous = landmark.previous;
+    Landmark? next = landmark.next;
     landmarks.remove(landmark);
+    // removing the landmark means we need to recompute the time between the two adjoined landmarks
+    if (previous != null && next != null) {
+      // previous.next = next happens automatically since we are using a LinkedList
+      previous.tripTime = null;
+      // TODO
+    }
     notifyListeners();
   }