demos - remove observable.dart import

Author: vaind

objectbox/example/flutter/objectbox_demo/lib/main.dart

@@ -5,7 +5,6 @@ import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 import 'package:path_provider/path_provider.dart';
 import 'package:objectbox/objectbox.dart';
-import 'package:objectbox/observable.dart';
 
 import 'objectbox.g.dart';
 

objectbox/example/flutter/objectbox_demo_sync/lib/main.dart

@@ -5,7 +5,6 @@ import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 import 'package:path_provider/path_provider.dart';
 import 'package:objectbox/objectbox.dart';
-import 'package:objectbox/observable.dart';
 
 import 'objectbox.g.dart';