remove unused enum SectorAttributes

forgotten in 67a939d31341b668b51fd24023c2a1d127b4d326
move m_bTaken from CMapTile into CSector

It can only be taken if planets exist.
refactor CMapTile::CalculateOwner
delete part of CMapTile::BuildStation

I think this is handled/overwritten by CalculateOwner() anyway.
move m_bMinor from CMapTile to CSector

Possible by previous commit.