[gameplay] Make siege engines uncapturable
Siege engines are faster to destroy than to capture. As such, capture as a default is annoying, since it's not the best tactical choice. Unfortunately, fixing this for now means removing "Capturable" from siege engines.
Rams/Siege towers retain 50 Pierce Armour, others get 25.
Patch by: Nescio
Differential Revision: https://code.wildfiregames.com/D2493