revert time onAddActivity

Author: sachaarbonel

packages/stream_feed_flutter_core/lib/src/bloc/feed_bloc.dart

@@ -92,7 +92,6 @@ class GenericFeedBloc<A, Ob, T, Or> {
       object: object,
       extraData: data,
       to: to,
-      time: DateTime.now(),
     );
 
     final flatFeed = client.flatFeed(feedGroup, userId);

packages/stream_feed_flutter_core/test/bloc/bloc_test.dart

@@ -61,11 +61,7 @@ void main() {
     ];
     when(() => mockClient.reactions).thenReturn(mockReactions);
 
-    activity = const Activity(
-      actor: 'test',
-      verb: 'post',
-      object: 'test',
-    );
+    activity = Activity(actor: 'test', verb: 'post', object: 'test');
 
     addedActivity = const Activity(
       id: 'test',