by newgrf files. When this is enabled forests cannot be built below the highest
snow line, and farms can't be built above it. Houses still use the
_opt.snow_line so they are all consistent, so to make them respect the snowline
you may want to use some newhouses features as well.
Remove unnecessary EnsureNoVehicle() checks because they are
- pointless per se (f.e. there are no vehicles on house tiles)
- done again (f.e. using CMD_LANDSCAPE_CLEAR just a few lines later)
