Made ore mines indestructible

This was a game-breaking problem because it can cause no cash stale mates and that is boring. I consider this a regression/bug, but in fact the community discussed whether this is a feature which provides valid tactics or can be exploited too easily http://www.sleipnirstuff.com/forum/viewtopic.php?f=82&t=16286 and our wise crowd decided for invulnerable ore mines.