It's because TrainingYard, Eyes and Valley are added to the Skirmish menu.
You can fix this easily. Go to your RenX directory -> UDKgame -> config. You there CNC Canyon, CNC Complex and the rest of the maps. Just remove the CNC Eyes, CNC TrainingYard And CNC Valley ini file
It won't mess up the game
It Will remove Eyes, TrainingYard and Valley from Skirmish menu, but You can play Walls again