Allow being an exit node disappear

[github44-1]

There was option Allow being an exit node, but it disappeared after update

[lidarbtc]

duplicate #6926

[devos50]

Indeed duplicate of #6926. Given the possible ramifications of enabling this option, we removed it from the user interface. Enabling the exit node functionality is still possible by toggling the appropriate option in the Tribler configuration file.

[vi]

1. What happens to profiles migrated from older Triber versions with the option enabled? Does it stay enabled or gets silently and automatically disabled?
2. Is it a good idea to still display value of exit node status in config window, even without actual checkbox?

Tribler configuration file

3. Is it documented somewhere? .Tribler/7.13/triblerd.conf is short and does not seem to have this option, whether enabled or disabled. Or is exit node mode became not officially supported for end users, i.e. intended for maintenance team only?

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Trying to guess the needed snippet from source code:

[tunnel_community]
    exitnode_enabled = True

Log messages like Dropping data packets, refusing to be an exit node suggest that it does not really work though.