clear current chunk and entity slice on chunk unload

ideally this should of never mattered, as it will only
be hit if you teleport out of an unloaded chunk...

But apparently some people are triggering this.

See #1223

Spigot-Server-Patches/Configurable-Chunk-Inhabited-Timer.patch

@@ -9,7 +9,7 @@ aspects of vanilla gameplay to this factor.
 For people who want all chunks to be treated equally, you can disable the timer.
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
-index e634c3afd..54f23ea75 100644
+index e634c3afdc..54f23ea75a 100644
 --- a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 @@ -0,0 +0,0 @@ public class PaperWorldConfig {
@@ -23,7 +23,7 @@ index e634c3afd..54f23ea75 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
-index d84cc9843..da7b59434 100644
+index 5940fc2883..9613a31b9d 100644
 --- a/src/main/java/net/minecraft/server/Chunk.java
 +++ b/src/main/java/net/minecraft/server/Chunk.java
 @@ -0,0 +0,0 @@ public class Chunk {