Remove ghoststone_active shadow on_construct

…continued from the last episode, https://github.com/Jeija/minetest-mod-mesecons/issues/72.
![minetestghostshadow6](https://f.cloud.github.com/assets/1996449/141175/0e6889a8-723d-11e2-87f5-18b4d2678e01.png)
This approximately copies some worldedit code that makes ghoststone dig neighbor air nodes when turned active. 
![minetestghostshadow7](https://f.cloud.github.com/assets/1996449/141176/12ac1098-723d-11e2-9fa6-8bfae1cc7ce4.png)
Can you find a better way of making an invisible node leave no shadow?