Remove `settlements_in_world` and `settlements.txt`

The global(!) table settlements_in_world` has no use, but is serialized and
saved to a file `settlements.txt` every time during generation of a new
village, adding more lag.  This commit removes all related code:
* Remove all instances of `settlements_in_world`.
* Remove `settlements.load()` and `settlements.save()`.

mods/MAPGEN/mcl_villages/buildings.lua

@@ -93,10 +93,6 @@ function settlements.create_site_plan(maxp, minp, pr)
 	-- go build settlement around center
 	if not center_surface then return false end
 
-	-- add settlement to list
-	table.insert(settlements_in_world, center_surface)
-	-- save list to file
-	settlements.save()
 	-- initialize all settlement_info table
 	local count_buildings, number_of_buildings, number_built = settlements.initialize_settlement_info(pr)
 	-- first building is townhall in the center

mods/MAPGEN/mcl_villages/const.lua

@@ -65,11 +65,6 @@ settlements.schematic_table = {
 	{name = "well",		mts = schem_path.."well.mts",		hwidth =  6, hdepth =  8, hheight =  6, hsize = 10, max_num = 0.045, rplc = basic_pseudobiome_villages },
 }
 
---
--- list of settlements, load on server start up
---
-settlements_in_world = {}
---
 --
 -- maximum allowed difference in height for building a sttlement
 --

mods/MAPGEN/mcl_villages/init.lua

@@ -10,7 +10,6 @@ dofile(settlements.modpath.."/paths.lua")
 --
 -- load settlements on server
 --
-settlements_in_world = settlements.load()
 settlements.grundstellungen()
 
 

mods/MAPGEN/mcl_villages/utils.lua

@@ -88,29 +88,6 @@ function settlements.check_distance(settlement_info, building_pos, building_size
 	return true
 end
 -------------------------------------------------------------------------------
--- save list of generated settlements
--------------------------------------------------------------------------------
-function settlements.save()
-	local file = io.open(minetest.get_worldpath().."/settlements.txt", "w")
-	if file then
-		file:write(minetest.serialize(settlements_in_world))
-		file:close()
-	end
-end
--------------------------------------------------------------------------------
--- load list of generated settlements
--------------------------------------------------------------------------------
-function settlements.load()
-	local file = io.open(minetest.get_worldpath().."/settlements.txt", "r")
-	if file then
-		local table = minetest.deserialize(file:read("*all"))
-		if type(table) == "table" then
-			return table
-		end
-	end
-	return {}
-end
--------------------------------------------------------------------------------
 -- fill chests
 -------------------------------------------------------------------------------
 function settlements.fill_chest(pos, pr)