Remove all meeting specific user info (i.e. the meeting_user object) when removing a user from a meeting

[MSoeb]

Description: If a user with a vote delegation is deleted from a meeting, the other user which is affected still has the reference of the vote delegation.

List with vote delegations

List with a deleted user

with one deleted users:

Reproduction:

1. open a meeting
2. open settings > participants > allow vote delegation
3. open participants > create two users > provide vote delegation to them
4. open participants > deleted one of the users with the vote delegation
5. check the other user -> vote delegation still active

What should happen:
If a user is deleted from a meeting, the vote delegation should be deleted from both users which are affected by it.

[rrenkert]

We want to remove all meeting specific data for the removed user.
@emanuelschuetze Please check if this is ok.

[jsangmeister]

@rrenkert @luisa-beerboom I think we should talk about what actually needs to be removed and what should not. I don't think the current approach is feasible.