Modified CarController doesn't disappear from the player configuration dialog

[Henrik Heimbuerger]

Reproduction steps:

1. Create one player using e.g. a driving agent.
2. Close the player settings dialog.
3. Open the driving agent dialog and modify the agent (doesn't matter how).
4. Try to start the race, the validation dialog will tell you that player 1 has no valid car controller.
5. Reopen the player settings dialog.

Expected:
The 'Controller' column is empty.

Actual:
The 'Controller' column still shows the driving agent. However, when editing the player, the 'Agent' combobox is empty.