Add flutter_lints and fix analyzer errors, dead code, and unused imports

lib/widgets/map/map_data_manager.dart

@@ -1,6 +1,5 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_map/flutter_map.dart';
-import 'package:flutter_map_animations/flutter_map_animations.dart';
 import 'package:latlong2/latlong.dart';
 
 import '../../models/osm_node.dart';