Add original code by PilzAdam:

@ forum post updated: June 08, 2013

LICENSE.txt

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+            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
+                    Version 2, December 2004
+
+ Copyright (C) 2004 Sam Hocevar <sam@hocevar.net>
+
+ Everyone is permitted to copy and distribute verbatim or modified
+ copies of this license document, and changing it is allowed as long
+ as the name is changed.
+
+            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. You just DO WHAT THE FUCK YOU WANT TO.

README.md

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+## Clean mod for [Minetest][]
+
+
+---
+### **Description:**
+
+Fork of [PilzAdam's ***clean*** mod][f.pilzadam] for Minetest.
+
+
+---
+### **Licensing:**
+
+[WTFPL](LICENSE.txt)
+
+
+[Minetest]: http://www.minetest.net/
+
+[f.pilzadam]: https://forum.minetest.net/viewtopic.php?t=2777

description.txt

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+A very simple mod that deletes unknown blocks and removes unknown entities.

init.lua

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+-- clean by PilzAdam
+-- LICENSE: WTFPL
+
+local old_nodes = {"mod:a", "mod:b"}
+local old_entities = {}
+
+for _,node_name in ipairs(old_nodes) do
+    minetest.register_node(":"..node_name, {
+        groups = {old=1},
+    })
+end
+
+minetest.register_abm({
+    nodenames = {"group:old"},
+    interval = 1,
+    chance = 1,
+    action = function(pos, node)
+        minetest.env:remove_node(pos)
+    end,
+})
+
+for _,entity_name in ipairs(old_entities) do
+    minetest.register_entity(":"..entity_name, {
+        on_activate = function(self, staticdata)
+            self.object:remove()
+        end,
+    })
+end

mod.conf

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+name = clean
+author = PilzAdam
+license = WTFPL