Fix rush light level check

It's the light level incident on the rush itself, not
in the space above, that we care about.  We also
don't need air above the rush, because there's air
IN the rush node.

mods/nc_flora/rushes.lua

@@ -95,10 +95,7 @@ local function rushcheck(pos)
 		return false
 	end
 
-	local above = {x = pos.x, y = pos.y + 1, z = pos.z}
-	if not (nodecore.air_equivalent(above)
-		and nodecore.can_grass_grow_under(above))
-	then return true end
+	if not nodecore.can_grass_grow_under(pos) then return true end
 
 	return subst, below
 end