Removed mapgen_air

The mapgen_air alias is only used in content_abm.cpp to facilitate melting and freezing, which makes no sense. Because air is defined in C++, not Lua (the Lua definition doesn't even properly override the C++ definition in C++ functions), it is always available, and doesn't need an alias for games (as games are unable to define their own version of air anyway). It seems like for both consistency and logical reasons, mapgen_air should be removed.