S4: Button to delete/remove more unit types

[Gizzmicbob]

If a donkey gets bugged, there is no way to kill/delete it.
If you accidentally build too many trade ships, there is no way to remove the extras.

[kdsystem1337]

Thanks for the report, the problem might be a check from the game itself, I think it makes more sense to solve the basic problem.

[kdsystem1337]

Destroy button coming see https://settlers-united.com/changelog/ with later version than 0.9.172 by https://github.com/fbreuss