Fix unregistering entities from unloading chunks

CraftBukkit caused a regression here by making unloading chunks not
have a ticket added and returning unloaded future.

This caused entities who were killed in same tick their chunk is unloading
to not be able to be removed from the chunk.

This then results in dead entities lingering in the Chunk.

Combine that with a buggy detail of the previous implementation of
the Dupe UUID patch, then this was the likely source of the "Ghost entities"

Spigot-Server-Patches/0453-Fix-unregistering-entities-from-unloading-chunks.patch

@@ -0,0 +1,35 @@
+From efaf7c2426c5bfd1970e5d9a01fb43d96ee1aab2 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Tue, 31 Mar 2020 03:01:45 -0400
+Subject: [PATCH] Fix unregistering entities from unloading chunks
+
+CraftBukkit caused a regression here by making unloading chunks not
+have a ticket added and returning unloaded future.
+
+This caused entities who were killed in same tick their chunk is unloading
+to not be able to be removed from the chunk.
+
+This then results in dead entities lingering in the Chunk.
+
+Combine that with a buggy detail of the previous implementation of
+the Dupe UUID patch, then this was the likely source of the "Ghost entities"
+
+diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
+index 9cb22eef56..f7a9b58db3 100644
+--- a/src/main/java/net/minecraft/server/WorldServer.java
++++ b/src/main/java/net/minecraft/server/WorldServer.java
+@@ -1482,9 +1482,9 @@ public class WorldServer extends World {
+     }
+ 
+     private void removeEntityFromChunk(Entity entity) {
+-        IChunkAccess ichunkaccess = this.getChunkAt(entity.chunkX, entity.chunkZ, ChunkStatus.FULL, false);
++        Chunk ichunkaccess = entity.getCurrentChunk(); // Paper - getChunkAt(x,z,full,false) is broken by CraftBukkit as it won't return an unloading chunk. Use our current chunk reference as this points to what chunk they need to be removed from anyways
+ 
+-        if (ichunkaccess instanceof Chunk) {
++        if (ichunkaccess != null) { // Paper
+             ((Chunk) ichunkaccess).b(entity);
+         }
+ 
+-- 
+2.25.1
+