delete old world_building_gun::operator()
delete old world_building_gun::operator()

No one was using it, and worldgen_from_tilespec is already
so fast that something that writes air or rock everywhere
is dominated by the costs of detecting interiorness and the like
(commenting out interiorness checking for the kinda trivial default
scenario gave me 125 ms to create about 160x160x800 tiles
[80 tile_widths in -x, -y, -z, +x, +y, +z from the initial center]
which could be a bit better but maintaining this bitrotting
function is not a way to make progress with that.)

elidupree/Lasercake,elidupree/Lasercake,Lasercake/Lasercake,Lasercake/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,idupree/Lasercake,elidupree/Lasercake,elidupree/Lasercake,idupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,elidupree/Lasercake,Lasercake/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake