objectbox_demo_relations: drop unused flutter_test dependency (#25)

Author: greenrobot-team

objectbox/example/flutter/objectbox_demo_relations/pubspec.yaml

@@ -16,8 +16,6 @@ dependencies:
   path: ^1.8.0 # flutter_test depends on path 1.8.0
 
 dev_dependencies:
-  flutter_test:
-    sdk: flutter
   flutter_lints: ^1.0.0
   build_runner: ^2.0.0
   objectbox_generator: any