System18: Remove discarded trains from the game

[vandamm]

Fixes #11264

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• Add the pins or archive_alpha_games label if this change will break existing games
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May require pins if in any game discarded train was bought

[roseundy]

Sorry, this is not the correct way to fix this.

Take a look at the variable DISCARDED_TRAINS

[roseundy]

Please look at the variable defined in the Game base class: DISCARDED_TRAINS
This just needs to be changed in the map method map_*_constants

[vandamm]

@roseundy sorry did not find this and found 1822 implementation instead.
Fixed to using DISCARDED_TRAINS = :remove in the base System18 because it's a base rule. Checked all implemented maps and none of them need to override it per the rules

[roseundy]

We are still archiving System18 games, not pinning them.