Kicking users from room has broken thread safety

[bdach]

Noticed that this didn't work while testing #31637, but then checked master and ended up discovering that it is unrelated.

Can be reproduced on staging:

1. Get two multiplayer users in one room
2. As host, try to kick the other user
3. Nothing happens
4. If you try point (2) again, server-side exceptions will fire

On the kicked client this is tripping thread safety assertions that only fire in debug:

cc @smoogipoo - this has nothing to do with your changes, but I am under the impression that you want full control over multiplayer happenings