Unreliable deletion of user stories in US_MANUALLY mode

[Dan1elBox]

To Reproduce
Steps to reproduce the behavior:

1. Create a 'Planning with User Stories' Session
2. Create 4-5 Stories
3. Delete some Stories
4. Rejoin the session and observe how not all user stories were reliably deleted

Expected behavior
Deleted user stories should also be reliably removed from the database object to ensure consistency is maintained when rejoining the session.

Additional context
Strange behavior can also occur when using the filter function before deleting a user story,... the wrong item may be deleted or the view may become inconsistent. This should definitely be taken into account when fixing the issue!!!