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These usually simplify the mesh into triangles, if these triangles are parallel then the 'delete->dissolve' should take care of the line in between. This may leave edges broken by vertices though, any altering of the perimeter of the shape will deform the face and be harder to keep under control. To avoid that you need to dissolve the vertices as well. If the diagonal is dividing a quadrilateral by it's existing vertices this shouldn't happen. However if they're involved in a change in direction detailing the face and are not parallel (which I can see plenty of in the roof there) they cannot be dissolved. Or maybe they can, I haven't done much dissolving in the new version, but if it is allowed it will likely give you the same problem as the one I mentioned above. Best course of action would be to just leave them as they are I'm not the best at explaining things x3 took a couple years for me to learn blender myself ah anyway, hope this helps

whaaat you can do that? haha nevermind what I said

Game engines use triangles instead of any other shape. So when you export for 0 A.D. your mesh get converted. In order to remove them, you can use ALT + J in face mode (you have to select all).